fix: backfill_phash runs on the long maintenance lane, not the scheduler's quick one (4411)

A whole-library rehash with a 35-minute limit was matched by the
maintenance.* glob. It held a scheduler process for its whole run, and the
minute ticks queued behind it. An exact-name route now sends it to
maintenance_long.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LVjrnpQjRgHdvq95rASoiR

tests/test_celery_routing.py

@@ -36,3 +36,17 @@ def test_queue_for_mirrors_external_to_download():
         celery.conf.task_routes["backend.app.tasks.external.*"]["queue"]
         == "download"
     )
+
+
+def test_backfill_phash_runs_on_the_long_lane():
+    """It lives in maintenance.py, so the quick-lane glob matches it too —
+    the router must pick the exact name. A 35-minute rehash on the scheduler
+    lane blocked the minute ticks behind it (2026-09-24)."""
+    route = celery.amqp.router.route(
+        {}, "backend.app.tasks.maintenance.backfill_phash",
+    )
+    assert route["queue"].name == "maintenance_long"
+    quick = celery.amqp.router.route(
+        {}, "backend.app.tasks.maintenance.recover_stalled_task_runs",
+    )
+    assert quick["queue"].name == "maintenance"