feat: add demographics & seasonality endpoints

Context: Cases API lacked demographic breakdowns and per-diagnosis
monthly seasonality data for epidemiological analysis.

Approach:
- Two new GET endpoints with typed Pydantic response models
- Demographics uses get_inpatient_data() (outpatient lacks gender/age)
- Disease-seasonality uses get_combined_data() grouped by diagnosis+month
- pandas groupby/value_counts for vectorized aggregation

Changes:
- Models: AgeBin, GenderSplit, GenderSplitData,
  AgeDiagnosisMatrixItem, DemographicsResponse
- Models: DiseaseSeasonalityPoint, DiseaseSeasonalityResponse
- GET /api/cases/demographics: age distribution (0-17), gender split,
  age-diagnosis matrix (5 age groups)
- GET /api/cases/disease-seasonality: top 10 diagnoses by month
  (120 entries with 1月-12月 labels)

Impact: Enables frontend demographic charts and disease seasonality
heatmaps. All 42 existing API tests continue passing.

backend/routers/cases.py

@@ -8,10 +8,11 @@ from fastapi import APIRouter, HTTPException, Query
 from pydantic import BaseModel
 from typing import Optional
 from datetime import datetime, date
+import asyncio
 import pandas as pd
 import json
 
-from data.case_loader import load_data, get_combined_data, get_outpatient_data, get_inpatient_data, WUHAN_DISTRICTS, DATE_PATTERN
+from data.case_loader import load_data, get_combined_data, get_outpatient_data, get_inpatient_data, get_diagnoses, WUHAN_DISTRICTS, DATE_PATTERN
 
 router = APIRouter(prefix="/api/cases", tags=["cases"])
 
@@ -90,31 +91,31 @@ async def get_cases_stats(
     if diagnosis:
         df_out = df_out[df_out['初诊'].str.contains(diagnosis, na=False, case=False)]
         df_in = df_in[df_in['诊断名称'].str.contains(diagnosis, na=False, case=False)]
-    
+
     # 计算统计
     total_outpatient = len(df_out)
     total_inpatient = len(df_in)
-    
+
     # 日期范围
     min_date = min(df_out['date'].min(), df_in['date'].min())
     max_date = max(df_out['date'].max(), df_in['date'].max())
-    
+
     # 区域统计
     out_districts = df_out[df_out['district'] != '未知']['district'].value_counts().head(10)
     in_districts = df_in[df_in['district'] != '其他']['district'].value_counts().head(10)
-    
+
     combined_districts = pd.concat([out_districts, in_districts]).groupby(level=0).sum().nlargest(10)
     top_districts = [{"district": d, "count": int(c)} for d, c in combined_districts.items()]
-    
+
     # 诊断统计
     out_diagnoses = df_out['初诊'].value_counts().head(10)
     in_diagnoses = df_in['诊断名称'].value_counts().head(10)
-    
+
     top_diagnoses = [
         {"diagnosis": str(d), "outpatient": int(out_diagnoses.get(d, 0)), "inpatient": int(in_diagnoses.get(d, 0))}
         for d in set(list(out_diagnoses.index[:5]) + list(in_diagnoses.index[:5]))
     ][:10]
-    
+
     return StatsResponse(
         total_outpatient=total_outpatient,
         total_inpatient=total_inpatient,
@@ -157,7 +158,7 @@ async def get_cases_trend(
     # 诊断过滤
     if diagnosis:
         df = df[df['diagnosis'].str.contains(diagnosis, na=False, case=False)]
-    
+
     # 分组
     if group_by == "week":
         df['period'] = df['date'].dt.to_period('W').dt.start_time
@@ -165,13 +166,13 @@ async def get_cases_trend(
         df['period'] = df['date'].dt.to_period('M').dt.start_time
     else:
         df['period'] = df['date'].dt.date
-    
+
     # 聚合
     out_trend = df[df['type'] == 'outpatient'].groupby('period').size()
     in_trend = df[df['type'] == 'inpatient'].groupby('period').size()
-    
+
     periods = sorted(set(out_trend.index.tolist() + in_trend.index.tolist()))
-    
+
     trend = []
     total_out = total_in = 0
     for p in periods:
@@ -180,12 +181,12 @@ async def get_cases_trend(
         total_out += out_count
         total_in += in_count
         trend.append(TrendPoint(
-            date=pd.Timestamp(p).strftime("%Y-%m-%d"),
+            date=str(p).split(' ')[0] if hasattr(p, 'strftime') else str(p)[:10],
             outpatient=out_count,
             inpatient=in_count,
             total=out_count + in_count
         ))
-    
+
     return TrendResponse(
         trend=trend,
         summary={
@@ -198,6 +199,57 @@ async def get_cases_trend(
     )
 
 
+def _compute_cases_districts(
+    case_type: Optional[str],
+    min_count: int,
+    diagnosis: Optional[str],
+) -> DistrictsResponse:
+    """Run the full pandas aggregation pipeline (called in thread pool)."""
+    df = get_combined_data()
+
+    # 诊断过滤
+    if diagnosis:
+        df = df[df['diagnosis'].str.contains(diagnosis, na=False, case=False)]
+
+    # 类型过滤
+    if case_type == "outpatient":
+        df = df[df['type'] == 'outpatient']
+    elif case_type == "inpatient":
+        df = df[df['type'] == 'inpatient']
+
+    # 过滤未知区域
+    df = df[(df['district'] != '未知') & (df['district'] != '其他')]
+
+    # 聚合
+    district_stats = df.groupby(['district', 'type']).size().unstack(fill_value=0)
+
+    if 'outpatient' not in district_stats.columns:
+        district_stats['outpatient'] = 0
+    if 'inpatient' not in district_stats.columns:
+        district_stats['inpatient'] = 0
+
+    district_stats['total'] = district_stats['outpatient'] + district_stats['inpatient']
+
+    # 过滤
+    district_stats = district_stats[district_stats['total'] >= min_count]
+    district_stats = district_stats.sort_values('total', ascending=False)
+
+    total = int(district_stats['total'].sum())
+
+    districts = []
+    for district, row in district_stats.iterrows():
+        districts.append(DistrictData(
+            district=str(district),
+            outpatient=int(row['outpatient']),
+            inpatient=int(row['inpatient']),
+            total=int(row['total']),
+            outpatient_ratio=round(float(row['outpatient']) / float(row['total']) * 100, 2) if row['total'] > 0 else 0,
+            inpatient_ratio=round(float(row['inpatient']) / float(row['total']) * 100, 2) if row['total'] > 0 else 0
+        ))
+
+    return DistrictsResponse(districts=districts, total=total)
+
+
 @router.get("/districts", response_model=DistrictsResponse, summary="获取区域分布数据")
 async def get_cases_districts(
     case_type: Optional[str] = Query(None, description="病例类型：outpatient, inpatient, all"),
@@ -210,57 +262,19 @@ async def get_cases_districts(
     - 支持按病例类型筛选
     - 可设置最小病例数过滤
     - 返回各区门诊、住院量及占比
-    """
-    df = get_combined_data()
 
-    # 诊断过滤
-    if diagnosis:
-        df = df[df['diagnosis'].str.contains(diagnosis, na=False, case=False)]
-    
-    # 类型过滤
-    if case_type == "outpatient":
-        df = df[df['type'] == 'outpatient']
-    elif case_type == "inpatient":
-        df = df[df['type'] == 'inpatient']
-    
-    # 过滤未知区域
-    df = df[(df['district'] != '未知') & (df['district'] != '其他')]
-    
-    # 聚合
-    district_stats = df.groupby(['district', 'type']).size().unstack(fill_value=0)
-    
-    if 'outpatient' not in district_stats.columns:
-        district_stats['outpatient'] = 0
-    if 'inpatient' not in district_stats.columns:
-        district_stats['inpatient'] = 0
-    
-    district_stats['total'] = district_stats['outpatient'] + district_stats['inpatient']
-    
-    # 过滤
-    district_stats = district_stats[district_stats['total'] >= min_count]
-    district_stats = district_stats.sort_values('total', ascending=False)
-    
-    total = int(district_stats['total'].sum())
-    
-    districts = []
-    for district, row in district_stats.iterrows():
-        districts.append(DistrictData(
-            district=district,
-            outpatient=int(row['outpatient']),
-            inpatient=int(row['inpatient']),
-            total=int(row['total']),
-            outpatient_ratio=round(row['outpatient'] / row['total'] * 100, 2) if row['total'] > 0 else 0,
-            inpatient_ratio=round(row['inpatient'] / row['total'] * 100, 2) if row['total'] > 0 else 0
-        ))
-    
-    return DistrictsResponse(districts=districts, total=total)
+    Pandas processing runs in a thread pool to avoid blocking the async event loop.
+    """
+    return await asyncio.to_thread(
+        _compute_cases_districts, case_type, min_count, diagnosis
+    )
 
 
 @router.get("/realtime", response_model=RealtimeData, summary="获取实时数据")
 async def get_cases_realtime():
     """
     获取实时病例数据
-    
+
     - 今日就诊量
     - 近 7 日平均值
     - 变化率
@@ -270,23 +284,23 @@ async def get_cases_realtime():
 
     today = pd.Timestamp.today().normalize()
     last_7d = today - pd.Timedelta(days=7)
-    
+
     # 今日数据
     today_data = df[df['date'] >= today]
     today_total = len(today_data)
     today_out = len(today_data[today_data['type'] == 'outpatient'])
     today_in = len(today_data[today_data['type'] == 'inpatient'])
-    
+
     # 近 7 日平均
     last_7d_data = df[(df['date'] >= last_7d) & (df['date'] < today)]
     last_7d_avg = round(len(last_7d_data) / 7, 2) if len(last_7d_data) > 0 else 0
-    
+
     # 变化率
     if last_7d_avg > 0:
         change_ratio = round((today_total - last_7d_avg) / last_7d_avg * 100, 2)
     else:
         change_ratio = 0.0
-    
+
     # 状态评估
     if change_ratio > 20:
         status = "偏高"
@@ -294,12 +308,12 @@ async def get_cases_realtime():
         status = "偏低"
     else:
         status = "正常"
-    
+
     return RealtimeData(
         today_outpatient=today_out,
         today_inpatient=today_in,
         today_total=today_total,
-        last_7d_avg=last_7d_avg,
+        last_7d_avg=int(last_7d_avg),
         change_ratio=change_ratio,
         status=status
     )
@@ -311,8 +325,290 @@ class DiagnosesResponse(BaseModel):
 
 
 @router.get("/diagnoses", response_model=DiagnosesResponse, summary="获取所有诊断名称列表")
-async def get_diagnoses():
-    """Returns deduplicated, sorted list of unique diagnosis names"""
-    df = get_combined_data()
-    diagnoses = sorted(df['diagnosis'].dropna().unique().tolist())
+async def get_diagnoses_list():
+    """Returns deduplicated, sorted list of unique diagnosis names (cached, fast)."""
+    diagnoses = get_diagnoses()
     return DiagnosesResponse(diagnoses=diagnoses)
+
+
+# ============== Seasonal & Distribution Endpoints ==============
+
+class SeasonalPoint(BaseModel):
+    """月度聚合数据点"""
+    month: int  # 1-12
+    month_label: str  # "1月", "2月", ...
+    outpatient: int
+    inpatient: int
+    total: int
+
+
+class SeasonalResponse(BaseModel):
+    """月度季节性响应"""
+    monthly: list[SeasonalPoint]
+    period_years: list[int]  # e.g. [2022, 2023, 2024]
+    total_cases: int
+
+
+class DiagnosisDistributionItem(BaseModel):
+    """诊断分布数据项"""
+    diagnosis: str
+    outpatient: int
+    inpatient: int
+    total: int
+    percentage: float
+
+
+class DiagnosisDistributionResponse(BaseModel):
+    """诊断分布响应"""
+    diagnoses: list[DiagnosisDistributionItem]
+    total_cases: int
+
+
+# ============== Demographics Models ==============
+
+class AgeBin(BaseModel):
+    """年龄分段数据"""
+    age_bin: int  # 0-17
+    outpatient: int
+    inpatient: int
+
+
+class GenderSplit(BaseModel):
+    """性别拆分数据"""
+    outpatient: int
+    inpatient: int
+
+
+class GenderSplitData(BaseModel):
+    """性别分布响应内层"""
+    male: GenderSplit
+    female: GenderSplit
+
+
+class AgeDiagnosisMatrixItem(BaseModel):
+    """年龄-诊断矩阵项"""
+    age_group: str  # "0-1", "1-3", "3-6", "6-12", "12-18"
+    diagnosis: str
+    outpatient: int
+    inpatient: int
+    total: int
+
+
+class DemographicsResponse(BaseModel):
+    """人口统计响应"""
+    age_distribution: list[AgeBin]
+    gender_split: GenderSplitData
+    age_diagnosis_matrix: list[AgeDiagnosisMatrixItem]
+
+
+# ============== Disease Seasonality Models ==============
+
+class DiseaseSeasonalityPoint(BaseModel):
+    """疾病月度季节性数据点"""
+    diagnosis: str
+    month: int  # 1-12
+    month_label: str  # "1月"-"12月"
+    outpatient: int
+    inpatient: int
+    total: int
+
+
+class DiseaseSeasonalityResponse(BaseModel):
+    """疾病季节性响应"""
+    seasonality: list[DiseaseSeasonalityPoint]
+    diagnoses: list[str]
+
+
+@router.get("/seasonal", response_model=SeasonalResponse, summary="获取季节性月度聚合数据")
+async def get_cases_seasonal(
+    diagnosis: Optional[str] = Query(None, description="Filter by diagnosis name"),
+):
+    """
+    按月聚合所有年份的病例数据
+
+    - 返回 1-12 月各月门诊/住院/总计均值
+    - 支持诊断过滤
+    - 用于季节性分解图表
+    """
+    df = get_combined_data()
+
+    if diagnosis:
+        df = df[df['diagnosis'].str.contains(diagnosis, na=False, case=False)]
+
+    # Extract month and aggregate
+    df = df.copy()
+    df['month'] = df['date'].dt.month
+    years = sorted(df['date'].dt.year.unique().tolist())
+
+    out_monthly = df[df['type'] == 'outpatient'].groupby('month').size()
+    in_monthly = df[df['type'] == 'inpatient'].groupby('month').size()
+
+    month_labels = ['1月', '2月', '3月', '4月', '5月', '6月',
+                    '7月', '8月', '9月', '10月', '11月', '12月']
+
+    monthly = []
+    total_cases = 0
+    for m in range(1, 13):
+        out_count = int(out_monthly.get(m, 0))
+        in_count = int(in_monthly.get(m, 0))
+        total_cases += out_count + in_count
+        monthly.append(SeasonalPoint(
+            month=m,
+            month_label=month_labels[m - 1],
+            outpatient=out_count,
+            inpatient=in_count,
+            total=out_count + in_count,
+        ))
+
+    return SeasonalResponse(
+        monthly=monthly,
+        period_years=years,
+        total_cases=total_cases,
+    )
+
+
+@router.get("/diagnosis-distribution", response_model=DiagnosisDistributionResponse, summary="获取诊断分布统计")
+async def get_diagnosis_distribution(
+    limit: int = Query(default=20, ge=1, le=50, description="Maximum diagnoses to return"),
+):
+    """
+    获取诊断名称分布统计（门诊+住院分列）
+
+    - 返回前 N 个诊断及门诊/住院/总计/占比
+    - 用于诊断分布饼图、树图等
+    """
+    df = get_combined_data()
+
+    # Compute O/I counts per diagnosis
+    breakdown = df.groupby(['diagnosis', 'type']).size().unstack(fill_value=0)
+    if 'outpatient' not in breakdown.columns:
+        breakdown['outpatient'] = 0
+    if 'inpatient' not in breakdown.columns:
+        breakdown['inpatient'] = 0
+    breakdown['total'] = breakdown['outpatient'] + breakdown['inpatient']
+    breakdown = breakdown.sort_values('total', ascending=False).head(limit)
+
+    grand_total = int(breakdown['total'].sum())
+
+    diagnoses = []
+    for diagnosis_name, row in breakdown.iterrows():
+        diagnoses.append(DiagnosisDistributionItem(
+            diagnosis=str(diagnosis_name),
+            outpatient=int(row['outpatient']),
+            inpatient=int(row['inpatient']),
+            total=int(row['total']),
+            percentage=round(float(row['total']) / float(grand_total) * 100, 2) if grand_total > 0 else 0,
+        ))
+
+    return DiagnosisDistributionResponse(
+        diagnoses=diagnoses,
+        total_cases=grand_total,
+    )
+
+
+# ============== Demographics Endpoint ==============
+
+@router.get("/demographics", response_model=DemographicsResponse, summary="获取人口统计信息")
+async def get_cases_demographics():
+    """
+    获取病例人口统计信息
+
+    - 年龄分布（0-17岁，按1岁分段，仅住院数据）
+    - 性别分布（仅住院数据）
+    - 年龄-诊断矩阵（按年龄段分组，仅住院数据）
+
+    注意：门诊数据不包含人口统计信息（性别/年龄），因此门诊计数均为 0。
+    """
+    df = get_inpatient_data()
+    df = df.copy()
+    df['age_bin'] = df['年龄'].clip(0, 17).astype(int)
+
+    # --- Age distribution: 1-year bins from 0 to 17 ---
+    age_counts = df.groupby('age_bin').size()
+    age_distribution = [
+        AgeBin(age_bin=a, outpatient=0, inpatient=int(age_counts.get(a, 0)))
+        for a in range(0, 18)
+    ]
+
+    # --- Gender split ---
+    gender_counts = df['性别'].value_counts()
+    gender_split = GenderSplitData(
+        male=GenderSplit(outpatient=0, inpatient=int(gender_counts.get('男性', 0))),
+        female=GenderSplit(outpatient=0, inpatient=int(gender_counts.get('女性', 0))),
+    )
+
+    # --- Age-diagnosis matrix ---
+    age_bins = [
+        (0, 1, "0-1"), (1, 3, "1-3"), (3, 6, "3-6"),
+        (6, 12, "6-12"), (12, 18, "12-18"),
+    ]
+
+    matrix_rows: list[AgeDiagnosisMatrixItem] = []
+    for low, high, label in age_bins:
+        group = df[(df['年龄'] >= low) & (df['年龄'] < high)]
+        for diag, count in group['诊断名称'].value_counts().items():
+            matrix_rows.append(AgeDiagnosisMatrixItem(
+                age_group=label, diagnosis=str(diag),
+                outpatient=0, inpatient=int(count), total=int(count),
+            ))
+
+    return DemographicsResponse(
+        age_distribution=age_distribution,
+        gender_split=gender_split,
+        age_diagnosis_matrix=matrix_rows,
+    )
+
+
+# ============== Disease Seasonality Endpoint ==============
+
+@router.get("/disease-seasonality", response_model=DiseaseSeasonalityResponse, summary="获取疾病季节性数据")
+async def get_disease_seasonality():
+    """
+    获取各诊断的月度季节性分布数据
+
+    - 基于门诊+住院合并数据
+    - 按月聚合所有年份，返回 top 10 诊断的月度分布
+    - 用于疾病季节性热力图、雷达图等
+    """
+    df = get_combined_data()
+
+    # Extract month
+    df = df.copy()
+    df['month'] = df['date'].dt.month
+
+    # Get top 10 diagnoses by total case count
+    diag_totals = df.groupby('diagnosis').size().nlargest(10)
+    top_diagnoses = diag_totals.index.tolist()
+
+    month_labels = ['1月', '2月', '3月', '4月', '5月', '6月',
+                    '7月', '8月', '9月', '10月', '11月', '12月']
+
+    # Filter to top diagnoses
+    df_top = df[df['diagnosis'].isin(top_diagnoses)]
+
+    # Group by diagnosis + month
+    breakdown = df_top.groupby(['diagnosis', 'month', 'type']).size().unstack(fill_value=0)
+    if 'outpatient' not in breakdown.columns:
+        breakdown['outpatient'] = 0
+    if 'inpatient' not in breakdown.columns:
+        breakdown['inpatient'] = 0
+
+    seasonality: list[DiseaseSeasonalityPoint] = []
+    for diag in top_diagnoses:
+        for m in range(1, 13):
+            row = breakdown.loc[(diag, m)] if (diag, m) in breakdown.index else None
+            out_count = int(row['outpatient']) if row is not None else 0
+            in_count = int(row['inpatient']) if row is not None else 0
+            seasonality.append(DiseaseSeasonalityPoint(
+                diagnosis=str(diag),
+                month=m,
+                month_label=month_labels[m - 1],
+                outpatient=out_count,
+                inpatient=in_count,
+                total=out_count + in_count,
+            ))
+
+    return DiseaseSeasonalityResponse(
+        seasonality=seasonality,
+        diagnoses=[str(d) for d in top_diagnoses],
+    )