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/main.py

@@ -1,9 +1,11 @@
 import logging
 from contextlib import asynccontextmanager
+
 from fastapi import FastAPI
-from sqlalchemy.ext.asyncio import create_async_engine, async_sessionmaker, AsyncSession
+from sqlalchemy.ext.asyncio import AsyncSession, async_sessionmaker, create_async_engine
+
+from app.api.router import api_router
 from app.config import settings
-from app.models.announcement import Base
 
 engine = create_async_engine(settings.database_url, echo=settings.debug)
 async_session = async_sessionmaker(engine, class_=AsyncSession, expire_on_commit=False)
@@ -20,7 +22,7 @@ async def lifespan(app: FastAPI):
         level=getattr(logging, settings.log_level),
         format="%(asctime)s [%(levelname)s] %(name)s: %(message)s",
     )
-    from app.scheduler.jobs import start_scheduler, shutdown_scheduler
+    from app.scheduler.jobs import shutdown_scheduler, start_scheduler
     start_scheduler()
     yield
     shutdown_scheduler()
@@ -35,7 +37,6 @@ app = FastAPI(
     redoc_url=None,
 )
 
-from app.api.router import api_router
 app.include_router(api_router)