Player: seek and volume on /api/player

seek {ms} runs on the Spotify session task like a seek from the app (buffer
flushed, app notified). volume {value}|{delta} sets the player volume and
pushes it to the session, so the app's slider follows; setRemoteVolume now
also runs on the session task.

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: GET and transport (play/pause/toggle/stop/next/prev) done. Next: seek/volume, source override/url.
+  - [ ] /api/player: GET, transport (play/pause/toggle/stop/next/prev), seek and volume done (the app follows). Next: progress bar (seek) in the web UI, then source override/url.
 - [ ] 8. Mono sum, gain, polish.
 
 ## Phase 2 (parked)