fix(tags): normalize task fails fast on lock + logs progress

normalize_tags_task ran to the 40-min hard limit with zero logs (operator-
flagged 2026-06-07). Cause: a per-group merge repoints series_page (via
_repoint_series_pages); during the wedged 0040 migration that held ACCESS
EXCLUSIVE on series_page, the merge's UPDATE blocked on that lock. The time-box
check is at the top of the group loop, so a statement blocked mid-group never
yields back to it — the task sat until the Celery hard kill. No logs because the
only log fired per *finished* group.

- Set lock_timeout=30s on the normalize session (opt-in server_settings on the
  async factory). A blocked merge now raises, the per-group handler rolls back +
  counts an error, and the loop continues — one stuck group can't strand the
  chunk, and the budget checkpoint stays effective.
- Log group count at start + a heartbeat every 25 groups, so a long/slow run is
  diagnosable instead of silent.

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

backend/app/tasks/_async_session.py

@@ -15,8 +15,15 @@ from sqlalchemy.pool import NullPool
 from ..config import get_config
 
 
-def async_session_factory():
-    """Return ``(sessionmaker, engine)`` bound to a fresh async engine."""
+def async_session_factory(*, server_settings: dict | None = None):
+    """Return ``(sessionmaker, engine)`` bound to a fresh async engine.
+
+    ``server_settings`` (optional) are applied by asyncpg as per-connection GUCs
+    on connect. Because NullPool opens a fresh real connection per checkout, every
+    transaction in the task inherits them — used to set ``lock_timeout`` so a
+    statement blocked on a lock fails fast instead of hanging to the Celery hard
+    limit (operator-flagged normalize_tags timeout 2026-06-07).
+    """
     cfg = get_config()
     # NullPool: this engine lives for ONE task (created + disposed per
     # asyncio.run loop), so intra-task connection pooling buys nothing and
@@ -26,5 +33,13 @@ def async_session_factory():
     # 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)
+    connect_args = {}
+    if server_settings:
+        connect_args["server_settings"] = {
+            k: str(v) for k, v in server_settings.items()
+        }
+    engine = create_async_engine(
+        cfg.database_url, future=True, poolclass=NullPool,
+        connect_args=connect_args,
+    )
     return async_sessionmaker(engine, class_=AsyncSession, expire_on_commit=False), engine