feat(spotify): orchestrate go-librespot backend from BotInstance (Stage 2 Task 7)

Construct one SpotifyController per bot (config.spotify + per-bot work/config
dirs under DATA_DIR, threaded via BotManager + index). resolveAndPlay now
routes spotify: sentinels through controller.ensureStarted/playTrack +
player.playPcmStream (falling back to the Stage-1 message when unavailable),
fences/pauses the sidecar on source transitions, advances via controller
"trackEnded", and delegates pause/resume/stop transport. Correction C4:
no re-attach on a spotify->spotify handoff (playPcmStream once across tracks,
no player.stop() — playPcmStream fences the prior ffmpeg internally); occupancy
auto-pause/resume + updateAutoPause + a new BotInstance.seek() (web seek route)
also delegate to the sidecar.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/web/api/player.ts

@@ -198,7 +198,7 @@ export function createPlayerRouter(
           .json({ error: "position must be a finite non-negative number" });
         return;
       }
-      bot.getPlayer().seek(position);
+      bot.seek(position);
       res.json({ message: `Seeked to ${Math.floor(position)}s`, seekOffset: position });
     } catch (err) {
       res.status(500).json({ error: (err as Error).message });