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

backend/app/tasks/geo_cache_cleanup.py

@@ -114,7 +114,13 @@ def register(app: FastAPI) -> None:
             ``app.state.scheduler`` will receive the job.
     """
     settings = get_effective_settings(app)
-    app.state.scheduler.add_job(
+    scheduler = getattr(app.state, "scheduler", None)
+    if scheduler is None:
+        # In tests or standalone usage, scheduler may not be on app.state yet.
+        # Use a no-op fallback — the heartbeat won't be registered but no crash.
+        log.warning("geo_cache_cleanup_no_scheduler")
+        return
+    scheduler.add_job(
         _run_cleanup_with_resources,
         trigger="interval",
         seconds=GEO_CLEANUP_INTERVAL,