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>

pipeline/lib/spotify-genre.py

@@ -54,9 +54,9 @@ def _load_spotify_creds():
                 continue
             k, v = line.split("=", 1)
             env[k] = v.strip().strip("'").strip('"')
-    return env["SPOTIFY_CLIENT_ID"], env["SPOTIFY_CLIENT_SECRET"]
+    return env["SPOTIFY_CLIENT_ID"], env["SPOTIFY_CLIENT_SECRET"], env.get("SPOTIFY_REFRESH_TOKEN") or None
 
-SPOTIFY_CID, SPOTIFY_CSEC = _load_spotify_creds()
+SPOTIFY_CID, SPOTIFY_CSEC, SPOTIFY_REFRESH = _load_spotify_creds()
 WHITELIST_FILE = f"{ALEMBIC_CONFIG_DIR}/pipeline/genres-whitelist.txt"
 MAX_GENRES = 3
 SEPARATOR = "; "
@@ -76,7 +76,7 @@ def load_whitelist():
 
 
 def _spotify_token():
-    return get_token(SPOTIFY_CID, SPOTIFY_CSEC)
+    return get_token(SPOTIFY_CID, SPOTIFY_CSEC, SPOTIFY_REFRESH)
 
 
 # Cache: artist_name → list[str] of Spotify genres (or None if not found)