Player: GET /api/player with Spotify now-playing info

Source, state, artist/title/album, position and duration (from cspot's
playback state, so it matches the app), volume and `can`. cspot patch 0004
adds read getters for the playback state and context.

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

CLAUDE.md

@@ -69,7 +69,7 @@ Repo: https://gitea.apointless.space/bsncubed/aes67-ESP32-P4
     - An old, long-idle session can get out of sync with the app: it sends empty Load frames ("No tracks in frame") instead of Pause/Play, so controls do nothing. Quitting and reopening Spotify on the Mac fixes it.
   - [x] failover (auto mode): no session or (with `failover_on_pause`) paused for `failover_delay_s` -> HLS; Spotify playing -> Spotify within ~1 s; 30 ms fades; HLS suspended while Spotify plays; cspot is held back (not drained) while it isn't Spotify's turn, so it resumes where it paused. Verified: no session -> HLS, play -> Spotify, pause stays (on_pause off), pause -> HLS after 5 s (on_pause on), play -> Spotify at the paused position.
     - Open: `aes67_tx resync` (TX 20-27 ms late) ~11 s after each switch to Spotify (seen 3 times); clicks at switches not yet checked in a recording; HLS start sometimes hits CDN read timeouts.
-  - [ ] /api/player
+  - [ ] /api/player: GET done (source, state, track, position as the app shows it, volume, can). Next: transport POSTs, seek/volume, source override/url.
 - [ ] 8. Mono sum, gain, polish.
 
 ## Phase 2 (parked)