Fix/db connection pool limit

backend/app/celery/celery_app.py

@@ -1,7 +1,7 @@
 import logging
 
 from celery import Celery
-from celery.signals import task_prerun, worker_process_init
+from celery.signals import task_failure, task_postrun, task_prerun, worker_process_init
 from kombu import Exchange, Queue
 
 from app.core.config import settings
@@ -27,6 +27,46 @@ def log_pool_status(task: "celery.Task", **_: object) -> None:  # type: ignore[n
         )
 
 
+@task_postrun.connect
+def log_pool_status_post(task: "celery.Task", **_: object) -> None:  # type: ignore[name-defined]
+    """Log DB connection pool state after each task completes (success or failure).
+
+    Compare with task_prerun log — if checked_out is the same or higher after
+    the task, connections were not returned and are leaking.
+    """
+    from app.core.db import engine
+    from sqlalchemy.pool import QueuePool
+
+    pool = engine.pool
+    if isinstance(pool, QueuePool):
+        logger.info(
+            f"[pool] POST task={task.name} checked_out={pool.checkedout()} "
+            f"size={pool.size()} overflow={pool.overflow()}"
+        )
+
+
+@task_failure.connect
+def log_pool_status_failure(
+    task_id: str, exception: Exception, sender: "celery.Task", **_: object  # type: ignore[name-defined]
+) -> None:
+    """Log DB connection pool state on task failure.
+
+    Failures are the most likely path for sessions to leak — exceptions can
+    bypass session cleanup if not guarded by a context manager.
+    """
+    from app.core.db import engine
+    from sqlalchemy.pool import QueuePool
+
+    pool = engine.pool
+    if isinstance(pool, QueuePool):
+        logger.warning(
+            f"[pool] FAILED task={sender.name} task_id={task_id} "
+            f"exc={type(exception).__name__} "
+            f"checked_out={pool.checkedout()} "
+            f"size={pool.size()} overflow={pool.overflow()}"
+        )
+
+
 @worker_process_init.connect
 def warm_llm_modules(**_) -> None:
     """Import LLM service modules in each worker process right after fork.