Player: transport commands on /api/player

play/pause/toggle/stop/next/prev. Spotify commands run on the session task
(same thread as cspot's frame handling) and the POST waits until they ran,
so it answers the new state. HLS pause stops fetching and resume rejoins at
the live edge; next/prev on HLS and anything on tone/off return 409.

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

main/hls.h

@@ -15,4 +15,5 @@ esp_err_t hls_start(hls_sink_t sink);
 // fetching runs only when no reason holds it.
 #define HLS_SUSPEND_OTA      (1 << 0)
 #define HLS_SUSPEND_FAILOVER (1 << 1)
+#define HLS_SUSPEND_USER     (1 << 2)     // paused via the player API
 void hls_suspend(unsigned reason, bool suspend);