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/v1/playlists.py

@@ -1,15 +1,18 @@
 """Playlist endpoints."""
 
 import uuid
-from typing import Any
 
 from fastapi import APIRouter, Query, Response
+from fastapi.responses import StreamingResponse
 
+from app.api.covers import stream_cover
 from app.api.deps import (
     AlbumRepoDep,
     ArtistRepoDep,
     CurrentUser,
+    FileStorageDep,
     PlaylistRepoDep,
+    StreamUser,
     TrackRepoDep,
 )
 from app.api.schemas.pagination import PagedResponse
@@ -217,4 +220,14 @@ async def reorder_playlist_tracks(
 
 
 @router.get("/{playlist_id}/cover")
-async def get_playlist_cover(playlist_id: uuid.UUID, _: CurrentUser) -> Any: ...
+async def get_playlist_cover(
+    playlist_id: uuid.UUID,
+    playlist_repo: PlaylistRepoDep,
+    storage: FileStorageDep,
+    _: StreamUser,
+) -> StreamingResponse:
+    # ``<img>`` can't send a bearer header → StreamUser accepts ``?token=``.
+    cover_path = await playlist_repo.get_cover_path(playlist_id)
+    if not cover_path:
+        raise NotFoundError("Cover not found.")
+    return await stream_cover(storage, cover_path)