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/domain/ports.py

@@ -29,6 +29,7 @@ from app.domain.entities import (
     SubsonicCredentials,
     User,
 )
+from app.domain.entities.settings import UserSettings
 from app.domain.entities.track import Artist, Track
 from app.domain.sources import DownloadResult, RawMetadata, SearchResult, SourceFile, SourceInfo
 from app.domain.tokens import IssuedToken, TokenClaims, TokenType
@@ -56,6 +57,11 @@ class UserRepository(Protocol):
     async def set_subsonic_password_enc(self, user_id: uuid.UUID, password_enc: str) -> None: ...
 
 
+class UserSettingsRepository(Protocol):
+    async def get(self, user_id: uuid.UUID) -> UserSettings | None: ...
+    async def upsert(self, settings: UserSettings) -> UserSettings: ...
+
+
 class SubsonicCipher(Protocol):
     """Symmetric encrypt/decrypt for the recoverable Subsonic app-password."""
 
@@ -164,6 +170,12 @@ class TrackRepository(Protocol):
     # AlbumRepository below).
     async def genres(self) -> list[tuple[str, int]]: ...
     async def library_stats(self) -> LibraryStats: ...
+    async def find_duplicate_groups(self) -> list[tuple[str, list[Track]]]: ...
+    async def list_by_metadata_status(
+        self, status: str, *, limit: int, offset: int
+    ) -> list[Track]: ...
+    async def all_storage_refs(self) -> list[tuple[uuid.UUID, str]]: ...
+    async def count_by_metadata_status(self, status: str) -> int: ...
     async def list(
         self,
         *,
@@ -287,6 +299,7 @@ class PlaylistRepository(Protocol):
     async def reorder_tracks(
         self, playlist_id: uuid.UUID, ordered_track_ids: list[uuid.UUID]
     ) -> None: ...
+    async def get_cover_path(self, playlist_id: uuid.UUID) -> str | None: ...
     # list must come after any method using list[...] in its signature (name shadowing)
     async def list(self, *, owner_id: uuid.UUID, limit: int, offset: int) -> list[Playlist]: ...