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/api/system_activity.py

@@ -31,7 +31,7 @@ system_activity_bp = Blueprint(
 # absent.
 _QUEUE_NAMES = (
     "default", "import", "thumbnail", "ml",
-    "download", "scan", "maintenance",
+    "download", "scan", "maintenance", "maintenance_long",
 )
 
 # Cache module-level so all requests share the cache between polls.