feat: add A-share and HK stock data via AKShare (TDD)

5 new endpoints under /api/v1/cn/:
- GET /cn/a-share/{symbol}/quote - A-share real-time quote
- GET /cn/a-share/{symbol}/historical - A-share historical OHLCV
- GET /cn/a-share/search?query= - search A-shares by name
- GET /cn/hk/{symbol}/quote - HK stock real-time quote
- GET /cn/hk/{symbol}/historical - HK stock historical OHLCV

Features:
- Chinese column names auto-mapped to English
- Symbol validation: A-share ^[036]\d{5}$, HK ^\d{5}$
- qfq (forward-adjusted) prices by default
- 79 new tests (51 service unit + 28 route integration)
- All 470 tests passing

pyproject.toml

@@ -10,6 +10,7 @@ dependencies = [
     "pydantic-settings",
     "httpx",
     "curl_cffi==0.7.4",
+    "akshare",
 ]
 
 [project.optional-dependencies]