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

backend/app/main.py

@@ -254,8 +254,9 @@ async def _lifespan(app: FastAPI) -> AsyncGenerator[None, None]:
         # task's coroutine handles cleanup.
         import asyncio  # noqa: TC003
 
+        current_task = asyncio.current_task()
         pending_tasks: list[asyncio.Task[Any]] = [
-            t for t in asyncio.all_tasks() if not t.done()
+            t for t in asyncio.all_tasks() if not t.done() and t is not current_task
         ]
         if pending_tasks:
             log.info(