fix: stabilize background operations and large embedding results

backend/app/retrieval/activity.py

@@ -0,0 +1,30 @@
+"""Process-local retrieval activity, shared by search, RAG and Agent callers."""
+import asyncio
+from functools import wraps
+
+active = 0
+completed = 0
+failed = 0
+cancelled = 0
+
+
+def track_search(operation):
+    @wraps(operation)
+    async def wrapped(self, request):
+        global active, completed, failed, cancelled
+        if request.mode == 'fts':
+            return await operation(self, request)
+        active += 1
+        try:
+            result = await operation(self, request)
+            completed += 1
+            return result
+        except asyncio.CancelledError:
+            cancelled += 1
+            raise
+        except Exception:
+            failed += 1
+            raise
+        finally:
+            active -= 1
+    return wrapped