CI: make builds work on China-based self-hosted Gitea runner

The self-hosted Windows runner can't reach github.com (GFW), so the previous
workflow failed cloning actions/checkout and would also fail downloading the
Flutter SDK and the sqlite3 native binary.

- Vendor the official, SHA-256-verified sqlite3 binaries under vendor/sqlite3/
  and select them via pubspec hooks.user_defines (source: test-sqlite3). Builds
  and tests now run fully offline — no GitHub download, no proxy, no
  LD_LIBRARY_PATH hack (removed .local-sqlite/).
- Consolidate CI into one Windows workflow: fetch actions from the gitea.com
  mirror, use the runner's pre-installed Flutter (no SDK download), and use the
  flutter-io.cn pub/Flutter mirrors. Server tests use the Tsinghua PyPI mirror.
- Document the offline build + China mirrors in README.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

pubspec.yaml

@@ -61,3 +61,15 @@ dev_dependencies:
 
 flutter:
   uses-material-design: true
+
+# Use vendored, hash-verified sqlite3 native binaries (committed under
+# vendor/sqlite3/) instead of downloading them from GitHub releases at build
+# time. This keeps builds fully local/offline — important behind the GFW where
+# the GitHub download times out — and deterministic. To support another
+# platform/arch, drop its official release binary into vendor/sqlite3/ (the
+# build validates each file's SHA-256 against the sqlite3 package).
+hooks:
+  user_defines:
+    sqlite3:
+      source: test-sqlite3
+      directory: vendor/sqlite3/