fix(db): reap orphaned idle-in-transaction sessions on the Celery engine

The long-running ingestion/podcast/video tasks run on a separate Celery
engine (NullPool), so the web engine's idle_in_transaction_session_timeout
did not cover them — which is exactly where the original 11h zombie
(INSERT INTO chunks) came from. Apply the same protection to the Celery
engine with a generous 60-minute default so a worker that hangs/crashes
mid-transaction can't hold locks on documents/chunks indefinitely, while
never reaping a legitimate per-document embed window.

- config + .env.example: DB_CELERY_IDLE_IN_TX_TIMEOUT_MS (default 3600000).

Co-authored-by: Cursor <cursoragent@cursor.com>

surfsense_backend/.env.example

@@ -10,6 +10,10 @@ DATABASE_URL=postgresql+asyncpg://postgres:postgres@localhost:5432/surfsense
 # idle_in_transaction_session_timeout (ms) so an abandoned "idle in transaction"
 # session can't wedge the DB indefinitely. 0 disables. (asyncpg only)
 # DB_IDLE_IN_TX_TIMEOUT_MS=900000
+# Same, for the Celery worker engine (long ingestion/podcast/video tasks). If a
+# task hasn't touched the DB in this window it's treated as orphaned and dropped.
+# 0 disables. (asyncpg only)
+# DB_CELERY_IDLE_IN_TX_TIMEOUT_MS=3600000
 
 # Deployment environment: dev or production
 SURFSENSE_ENV=dev