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>

tests/test_celery_routing.py

@@ -0,0 +1,20 @@
+"""Queue routing — the long one-shot maintenance tasks run on a dedicated
+`maintenance_long` lane so a 30-min backup or a multi-chunk audit can't starve
+the quick recovery sweeps / vacuum on the concurrency-1 `maintenance` lane
+(operator-flagged 2026-06-07)."""
+from backend.app.celery_app import celery
+
+
+def test_long_one_shots_route_to_maintenance_long():
+    routes = celery.conf.task_routes
+    for prefix in (
+        "backend.app.tasks.backup.*",
+        "backend.app.tasks.admin.*",
+        "backend.app.tasks.library_audit.*",
+    ):
+        assert routes[prefix]["queue"] == "maintenance_long"
+
+
+def test_quick_maintenance_stays_on_maintenance():
+    routes = celery.conf.task_routes
+    assert routes["backend.app.tasks.maintenance.*"]["queue"] == "maintenance"