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/build-fingerprint-index.py

@@ -189,7 +189,7 @@ def main() -> int:
     elapsed = time.time() - start
     print(f"[index] done in {elapsed:.0f}s. wrote {done - failed} rows, {failed} failed. "
           f"index now has {total} entries.", flush=True)
-    return 0 if failed == 0 else 1
+    return 0 if (done - failed) > 0 else 1
 
 
 if __name__ == "__main__":