Refactor backend: fix geo cache cleanup, scheduler heartbeat, correlation middleware; update docs

backend/app/startup.py

@@ -459,6 +459,8 @@ async def _stage_register_tasks(app: FastAPI, scheduler: AsyncIOScheduler) -> No
         app: The FastAPI application instance.
         scheduler: The APScheduler scheduler to register tasks with.
     """
+    # Set scheduler on app.state before registering tasks (they use app.state.scheduler)
+    app.state.scheduler = scheduler
     scheduler_lock_heartbeat.register(app)
     health_check.register(app)
     await blocklist_import.register(app)