Remove dead task DB fast-path and update task tests

backend/app/tasks/geo_cache_flush.py

@@ -34,10 +34,6 @@ JOB_ID: str = "geo_cache_flush"
 
 
 async def _get_db(app: Any) -> tuple[aiosqlite.Connection, bool]:
-    existing_db = getattr(app.state, "db", None)
-    if existing_db is not None:
-        return existing_db, False
-
     db = await open_db(app.state.settings.database_path)
     return db, True