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_auth.py

@@ -12,19 +12,27 @@ import urllib.parse
 import urllib.request
 
 _TOKEN_URL = "https://accounts.spotify.com/api/token"
-_cache: dict[tuple[str, str], str] = {}
+_cache: dict[tuple[str, str, str | None], str] = {}
 
 
-def get_token(client_id: str, client_secret: str) -> str:
-    """Return a client-credentials access token, cached per (id, secret) for
-    the life of the process."""
-    key = (client_id, client_secret)
+def get_token(client_id: str, client_secret: str, refresh_token: str | None = None) -> str:
+    """Return an access token, cached per (id, secret, refresh_token) for the
+    life of the process. If refresh_token is set (an account has been
+    connected via /connect/spotify), mints a user token -- required for
+    playlist reads on newly created Spotify apps, which reject
+    client-credentials tokens. Otherwise falls back to client-credentials."""
+    key = (client_id, client_secret, refresh_token)
     if key in _cache:
         return _cache[key]
     creds = base64.b64encode(f"{client_id}:{client_secret}".encode()).decode()
+    grant = (
+        {"grant_type": "refresh_token", "refresh_token": refresh_token}
+        if refresh_token
+        else {"grant_type": "client_credentials"}
+    )
     req = urllib.request.Request(
         _TOKEN_URL,
-        data=urllib.parse.urlencode({"grant_type": "client_credentials"}).encode(),
+        data=urllib.parse.urlencode(grant).encode(),
         headers={
             "Authorization": f"Basic {creds}",
             "Content-Type": "application/x-www-form-urlencoded",