0.6: Spotify OAuth connect flow, AzuraCast base URL, fingerprint/buy-url fixes

- Add "Connect Spotify" OAuth flow (/connect/spotify) so newly created Spotify
  apps can read playlists: Spotify now requires a user token for playlist
  reads, which client-credentials alone can no longer provide. The stored
  refresh token is rendered as spotify-refresh into each playlist's sldl
  .conf -- sldl's own docs confirm supplying it skips its interactive login
  flow, which is what was causing multi-hour hangs on a stuck sync.
  Grandfathered apps keep working unchanged if no account is connected.
- Fix the connect flow's redirect_uri: request.url_for() reflected the raw
  connection scheme (http) rather than what the reverse proxy actually
  served over, which Spotify's exact-match redirect_uri check rejected.
  Force https rather than trust the unproxied scheme.
- Add an AzuraCast base URL credential field alongside the API key, with a
  keyfile fallback so the scheduled enrich-buy-url.py job can actually reach
  AzuraCast (previously it had no way to receive a base URL at all when run
  from the scheduler, so the reprocess call was silently always skipped).
- Fix build-fingerprint-index.py exiting non-zero on any single fingerprint
  failure instead of only when nothing was written.
- Guard enrich-buy-url.py's AzuraCast reprocess against an empty
  --azuracast-base (raised ValueError since PD2 removed the hardcoded base).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

app/main.py

@@ -9,7 +9,7 @@ from starlette.exceptions import HTTPException as StarletteHTTPException
 from starlette.middleware.sessions import SessionMiddleware
 
 from app.db import enable_beets_db_wal, init_db, mark_interrupted_runs
-from app.routers import artist_casing, auth, credentials, dashboard, dedup, genres, health, import_, jobs, library, playlists
+from app.routers import artist_casing, auth, connect, credentials, dashboard, dedup, genres, health, import_, jobs, library, playlists
 from app.security.session import resolve_session_secret
 from app.services import scheduler_service
 from app.settings import settings
@@ -82,6 +82,7 @@ def create_app() -> FastAPI:
     app.include_router(genres.router)
     app.include_router(playlists.router)
     app.include_router(credentials.router)
+    app.include_router(connect.router)
     app.include_router(jobs.router)
     app.include_router(artist_casing.router)