fix: prevent subscription worker deadlocks

backend/db/advisory_locks.py

@@ -0,0 +1,12 @@
+from sqlalchemy import text
+from sqlalchemy.ext.asyncio import AsyncSession
+
+# Serializes background jobs that rewrite subscription rows from panel state.
+SUBSCRIPTION_BACKGROUND_SYNC_LOCK_ID = 817512404897421338
+
+
+async def acquire_subscription_background_sync_lock(session: AsyncSession) -> None:
+    await session.execute(
+        text("SELECT pg_advisory_xact_lock(:lock_id)"),
+        {"lock_id": SUBSCRIPTION_BACKGROUND_SYNC_LOCK_ID},
+    )