fix: analysis 500s, caching, alert page perf

P0: Fix KeyError in 3 analysis endpoints. geojson.py stores 1d risk
as "risk_value" but analysis.py accessed "risk_1d" — always crashed.

Backend: Add lru_cache to GeoJSON/CSV/Parquet loaders, date helpers,
and district loader. Add try/except and FileNotFoundError guards.

Frontend: Debounce riskRange, merge counts into useMemo, stabilize
handleGridClick with ref, memoize nearest-grid scan, wrap AlertMap
in React.memo, switch useLodGrid from fetch to cachedGet.

backend/routers/geocoded.py

@@ -5,6 +5,7 @@ from fastapi import APIRouter, HTTPException
 from pydantic import BaseModel
 from typing import List, Optional
 import logging
+from functools import lru_cache
 import pandas as pd
 from pathlib import Path
 
@@ -15,6 +16,11 @@ router = APIRouter(prefix="/api/geocoded", tags=["geocoded"])
 PROJECT_ROOT = Path(__file__).parent.parent.parent
 DATA_DIR = PROJECT_ROOT / "outputs"
 
+
+@lru_cache(maxsize=1)
+def _load_csv(path: Path) -> pd.DataFrame:
+    return pd.read_csv(path)
+
 class GridCaseData(BaseModel):
     """Grid case data for visualization"""
     grid_id: int
@@ -60,7 +66,7 @@ async def get_grid_cases():
         raise HTTPException(status_code=404, detail="Grid data not found")
     
     try:
-        df = pd.read_csv(grid_file)
+        df = _load_csv(grid_file)
         
         grids = []
         for _, row in df.iterrows():
@@ -105,7 +111,7 @@ async def get_geocoded_cases(
         raise HTTPException(status_code=404, detail="Geocoded data not found")
     
     try:
-        df = pd.read_csv(cases_file)
+        df = _load_csv(cases_file)
 
         # Drop rows with missing coordinates
         df = df.dropna(subset=['latitude', 'longitude'])
@@ -122,7 +128,7 @@ async def get_geocoded_cases(
         df = df.head(limit)
 
         cases = []
-        for _, row in df.iterrows():
+        for row in df.to_dict('records'):
             street_val = row.get('street')
             if pd.isna(street_val):
                 street_val = None
@@ -157,7 +163,7 @@ async def get_geocoded_count():
         raise HTTPException(status_code=404, detail="Geocoded data not found")
     
     try:
-        df = pd.read_csv(cases_file)
+        df = _load_csv(cases_file)
         street_matched = len(df[df['geocode_method'] == 'street'])
         district_fallback = len(df[df['geocode_method'] == 'district'])