feat(maintenance): dedicated maintenance_long lane for long one-shot tasks

Even chunked, a single concurrency-1 maintenance lane is fragile — a 30-min DB
backup or a multi-chunk library audit holds the slot and delays the quick
self-healing recovery sweeps / vacuum (operator-flagged 2026-06-07: long runs
must never block quick maintenance).

Route the long one-shots — backup.*, admin.* (normalize/re-extract/cascade-
delete), library_audit.* — to a new `maintenance_long` queue served by a
dedicated worker (concurrency 1), added to docker-compose (+ dev override). The
scheduler keeps the quick `maintenance` lane (sweeps, vacuum, cleanup) for
itself, so a backup can no longer starve a 5-min vacuum. UI queue list +
routing tests updated.

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

backend/app/celery_app.py

@@ -43,10 +43,16 @@ def make_celery() -> Celery:
             "backend.app.tasks.thumbnail.*": {"queue": "thumbnail"},
             "backend.app.tasks.download.*": {"queue": "download"},
             "backend.app.tasks.scan.*": {"queue": "scan"},
+            # `maintenance` is the QUICK lane — recovery sweeps, vacuum, cleanup
+            # (concurrency-1 on the scheduler). The long one-shots (DB backups,
+            # library audits, admin maintenance: normalize/re-extract/cascade-
+            # delete) run on a SEPARATE `maintenance_long` lane + worker so they
+            # can never starve the quick self-healing sweeps (operator-flagged
+            # 2026-06-07: a 2h audit blocked vacuum/backup/normalize for hours).
             "backend.app.tasks.maintenance.*": {"queue": "maintenance"},
-            "backend.app.tasks.backup.*": {"queue": "maintenance"},
-            "backend.app.tasks.admin.*": {"queue": "maintenance"},
-            "backend.app.tasks.library_audit.*": {"queue": "maintenance"},
+            "backend.app.tasks.backup.*": {"queue": "maintenance_long"},
+            "backend.app.tasks.admin.*": {"queue": "maintenance_long"},
+            "backend.app.tasks.library_audit.*": {"queue": "maintenance_long"},
         },
         # Heavy ML tasks need fair dispatch — see ImageRepo's precedent.
         task_acks_late=True,