feat(api): finish Group A stubbed endpoints

Implements previously-stubbed /api/v1 endpoints (hexagonal: ports -> repos
-> services -> routers wired in deps):

- playlists: GET /{id}/cover (serves stored cover, 404 when absent)
- settings: GET/PATCH /settings + GET/PUT /settings/scrobbling — lazy
  per-user row, write-only Fernet-encrypted scrobble session key; adds
  user_settings table + migration (chains off dc126696f5a6)
- storage: GET /duplicates, /broken, /missing-metadata + admin POST
  /cleanup (arq cleanup_storage worker; reconciles local refs only,
  guarded against a storage-outage mass delete)
- admin: GET /services, /sources, /settings + POST /reindex; PATCH
  /settings and /sources/{source} return 501 (config is env-managed)

Adds NotSupportedError (-> HTTP 501). Integration tests for each surface.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

app/workers/arq_worker.py

@@ -9,6 +9,7 @@ from arq.connections import RedisSettings
 
 from app.core.config import get_settings
 from app.core.logging import configure_logging, get_logger
+from app.workers.tasks.cleanup_task import cleanup_storage
 from app.workers.tasks.download_task import download_track
 from app.workers.tasks.enrich_task import enrich_track
 from app.workers.tasks.import_task import scan_local_folder
@@ -33,6 +34,7 @@ class WorkerSettings:
         enrich_track,
         download_track,
         materialize_track,
+        cleanup_storage,
     ]
     on_startup = startup
     on_shutdown = shutdown