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/schemas/settings.py

@@ -0,0 +1,49 @@
+"""User-settings request/response schemas.
+
+Enums are enforced at the API boundary (Pydantic ``Literal`` → 422 on bad
+input), so the service can trust the values it receives.
+"""
+
+from typing import Literal
+
+from pydantic import BaseModel, model_validator
+
+Theme = Literal["system", "light", "dark"]
+# Playback quality preference. ``original`` = no transcode; the lower tiers are
+# consumed by the (upcoming) transcoding pipeline.
+StreamQuality = Literal["original", "high", "medium", "low"]
+ScrobbleProvider = Literal["lastfm", "listenbrainz"]
+
+
+class SettingsOut(BaseModel):
+    theme: Theme
+    stream_quality: StreamQuality
+
+
+class SettingsUpdate(BaseModel):
+    """Partial update — omitted fields keep their current value."""
+
+    theme: Theme | None = None
+    stream_quality: StreamQuality | None = None
+
+
+class ScrobblingOut(BaseModel):
+    enabled: bool
+    provider: ScrobbleProvider | None
+    username: str | None
+    # Whether a session key is stored. The key itself is never returned.
+    configured: bool
+
+
+class ScrobblingUpdate(BaseModel):
+    enabled: bool = False
+    provider: ScrobbleProvider | None = None
+    username: str | None = None
+    # Write-only scrobbler session key / user token. Omit to keep the stored one.
+    session_key: str | None = None
+
+    @model_validator(mode="after")
+    def _provider_required_when_enabled(self) -> ScrobblingUpdate:
+        if self.enabled and self.provider is None:
+            raise ValueError("provider is required when scrobbling is enabled")
+        return self