fix: guard pool initialization with asyncio.Lock to prevent race condition

backend/jobs.py

@@ -1,5 +1,6 @@
 """Job definitions and enqueueing utilities using pgqueuer."""
 
+import asyncio
 import json
 
 import asyncpg
@@ -12,13 +13,20 @@ JOB_RANDOM_NUMBER = "random_number"
 
 # Connection pool for job enqueueing (lazy initialized)
 _pool: asyncpg.Pool | None = None
+_pool_lock = asyncio.Lock()
 
 
 async def get_job_pool() -> asyncpg.Pool:
     """Get or create the connection pool for job enqueueing."""
     global _pool
+    if _pool is not None:
+        return _pool
+    async with _pool_lock:
+        # Double-check after acquiring lock
         if _pool is None:
-        _pool = await asyncpg.create_pool(ASYNCPG_DATABASE_URL, min_size=1, max_size=5)
+            _pool = await asyncpg.create_pool(
+                ASYNCPG_DATABASE_URL, min_size=1, max_size=5
+            )
         return _pool