Player: auto mode fails over between Spotify and HLS

No Spotify session (or paused, with failover_on_pause) for failover_delay_s
switches to HLS; Spotify playing switches back within ~1 s. Switches fade
out/in over 30 ms and flush the ring. HLS is suspended while Spotify plays.
cspot is held back instead of drained while it isn't Spotify's turn: it keeps
decoding on pause, so dropping its frames let it race through the queue.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

components/spotify/include/spotify.h

@@ -40,6 +40,8 @@ void spotify_suspend(bool suspend);
 // The player's volume (0..100 %): reported to the Spotify app when a session starts (else the app
 // shows 0 while we play at the player's level), and pushed to a running session.
 void spotify_set_volume(int pct);
+// A session is running (an app is connected to this device).
+bool spotify_session_active(void);
 // Current state for status.spotify_state.
 const char *spotify_state(void);