chore: ruff 代码检查与修复

130 issues auto-fixed (import ordering, UP045/UP006 type annotations),
33 issues manually fixed (E712/E501/E402/E722 + N818 rename + per-file
wechat ignore for N8xx naming conventions). All 33 tests pass.

app/wechat/client.py

@@ -1,16 +1,16 @@
 import time
+
 import httpx
-from typing import Optional
 
 from app.config import settings
 
 
 class WeChatClient:
     def __init__(self):
-        self._access_token: Optional[str] = None
+        self._access_token: str | None = None
         self._token_expires_at: float = 0
 
-    async def _get_access_token(self) -> Optional[str]:
+    async def _get_access_token(self) -> str | None:
         now = time.time()
         if self._access_token and now < self._token_expires_at:
             return self._access_token