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>

docker-compose.yml

@@ -98,6 +98,25 @@ services:
       postgres: { condition: service_healthy }
       redis:    { condition: service_healthy }
 
+  # Dedicated lane for long one-shot maintenance (DB backups, library audits,
+  # admin maintenance). Kept off the scheduler's quick `maintenance` lane so a
+  # 30-min backup or a multi-chunk audit can never starve the 5-min recovery
+  # sweeps / vacuum (operator-flagged 2026-06-07). One slot — these are heavy.
+  maintenance-long:
+    image: git.fabledsword.com/bvandeusen/fabledcurator:dev
+    command: ["worker"]
+    environment:
+      <<: *app_env
+      CELERY_QUEUES: maintenance_long
+      CELERY_CONCURRENCY: "1"
+    volumes:
+      - ./images:/images
+      - ./import:/import
+      - ./downloads:/downloads
+    depends_on:
+      postgres: { condition: service_healthy }
+      redis:    { condition: service_healthy }
+
   ml-worker:
     image: git.fabledsword.com/bvandeusen/fabledcurator-ml:dev
     command: ["ml-worker"]