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/api/deps.py

@@ -22,6 +22,7 @@ from app.application.streaming_service import StreamingService
 from app.application.subsonic_auth_service import SubsonicAuthService
 from app.application.upload_service import UploadService
 from app.application.user_service import UserService
+from app.application.user_settings_service import UserSettingsService
 from app.core.config import get_settings
 from app.core.security import Argon2PasswordHasher, JwtTokenService, SubsonicPasswordCipher
 from app.domain.entities import User
@@ -38,6 +39,7 @@ from app.infrastructure.db.repositories import (
     SqlAlchemyRefreshTokenRepository,
     SqlAlchemyTrackRepository,
     SqlAlchemyUserRepository,
+    SqlAlchemyUserSettingsRepository,
 )
 from app.infrastructure.metadata.acoustid import AcoustIdHttpClient
 from app.infrastructure.metadata.fingerprint import FpcalcFingerprinter
@@ -112,9 +114,17 @@ def get_subsonic_auth_service(session: SessionDep) -> SubsonicAuthService:
     )
 
 
+def get_user_settings_service(session: SessionDep) -> UserSettingsService:
+    return UserSettingsService(
+        settings=SqlAlchemyUserSettingsRepository(session),
+        cipher=get_subsonic_cipher(),
+    )
+
+
 AuthServiceDep = Annotated[AuthService, Depends(get_auth_service)]
 UserServiceDep = Annotated[UserService, Depends(get_user_service)]
 SubsonicAuthServiceDep = Annotated[SubsonicAuthService, Depends(get_subsonic_auth_service)]
+UserSettingsServiceDep = Annotated[UserSettingsService, Depends(get_user_settings_service)]
 
 
 # -- file storage (process-cached) ---------------------------------------------