fix(download): release DB connections across the gallery-dl subprocess

Backfill events were STILL stranding empty after the timeout-ladder fix.
Worker logs showed the salvage path working ("Download timeout for
anduo/patreon after 1170.0s (18 files written)") but then:
  Retry in 3s: DBAPIError(ConnectionDoesNotExistError: connection was
  closed in the middle of operation)
  ...succeeded in 0.149s   <- in-flight guard no-op

Root cause: DownloadService held the async + sync DB connections checked
out across the entire (≤19.5-min backfill) gallery-dl subprocess. The
server reaps the idle connection, so phase 3's first query hits a dead
socket. That DBAPIError trips download_source's autoretry_for, the retry
re-enters _phase1_setup, sees the event still 'running', returns
in_flight and no-ops — leaving the event to be stranded empty by the
recovery sweep. pool_pre_ping was already on both engines but can't help
a *held* connection (it only validates on pool checkout).

Fix:
- DownloadService.download_source closes the async + sync sessions after
  phase 1, before the subprocess, so phase 3 re-acquires a live
  connection (matches the class's "Phase 2 — no DB connection" docstring).
- The per-task async engine switches to NullPool so phase 3 always opens
  a fresh connection rather than a pooled one the server may have reaped.

Tests: assert connections are released before gdl.download runs and the
event still finalizes; assert the task engine uses NullPool. Also fixes a
stale 1800s->1170s comment.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/app/services/download_service.py

@@ -91,10 +91,25 @@ class DownloadService:
             return setup["event_id"]
         ctx = setup
 
+        # Release the phase-1 DB connections before the (up to ~19.5-min in
+        # backfill) gallery-dl subprocess. Held checked-out across that idle
+        # window, the asyncpg/psycopg connections get reaped by the server,
+        # and phase 3's first query then hits a dead socket
+        # (asyncpg ConnectionDoesNotExistError) → download_source autoretry →
+        # _phase1_setup's in-flight guard no-ops the retry → the event
+        # strands empty for the recovery sweep (Anduo #40014, 2026-06-04).
+        # pool_pre_ping can't help a *held* connection — it only validates on
+        # pool checkout. Closing returns them to the pool so phase 3 re-
+        # acquires a live one (the async task engine uses NullPool, the sync
+        # engine pre_ping + pool_recycle=300). This is what makes the
+        # "Phase 2 — no DB connection" contract in the class docstring true.
+        await self.async_session.close()
+        self.sync_session.close()
+
         source_config = SourceConfig.from_dict(ctx["config_overrides"] or {})
         # alembic 0031 / plan #544: derive skip_value + timeout from the
         # source's backfill_runs_remaining counter. When > 0, walk the full
-        # post history (skip: True + 1800s); when 0, exit gallery-dl after
+        # post history (skip: True + 1170s); when 0, exit gallery-dl after
         # 20 contiguous archived items (skip: "exit:20" + the default
         # 870s). Operator sets backfill via POST /api/sources/{id}/backfill.
         backfill_remaining = ctx.get("backfill_runs_remaining", 0) or 0

backend/app/tasks/_async_session.py

@@ -10,6 +10,7 @@ disposes it (``await engine.dispose()``) when its loop ends.
 """
 
 from sqlalchemy.ext.asyncio import AsyncSession, async_sessionmaker, create_async_engine
+from sqlalchemy.pool import NullPool
 
 from ..config import get_config
 
@@ -17,5 +18,13 @@ from ..config import get_config
 def async_session_factory():
     """Return ``(sessionmaker, engine)`` bound to a fresh async engine."""
     cfg = get_config()
-    engine = create_async_engine(cfg.database_url, future=True, pool_pre_ping=True)
+    # NullPool: this engine lives for ONE task (created + disposed per
+    # asyncio.run loop), so intra-task connection pooling buys nothing and
+    # actively bit us — download_source releases its phase-1 connection
+    # before a multi-minute gallery-dl subprocess, and a *pooled* idle
+    # connection would be reaped by the server and handed back dead to
+    # phase 3 (asyncpg ConnectionDoesNotExistError, Anduo #40014). NullPool
+    # opens a fresh real connection on each checkout, so phase 3 always
+    # reconnects clean; pre_ping is then redundant.
+    engine = create_async_engine(cfg.database_url, future=True, poolclass=NullPool)
     return async_sessionmaker(engine, class_=AsyncSession, expire_on_commit=False), engine