Add callbacks support for all media actions

- Add CallbackConfig model for callback scripts
- Add callbacks section to config for optional command execution
- Add turn_on/turn_off/toggle endpoints (callback-only)
- Add callbacks for all media actions (play, pause, stop, next, previous, volume, mute, seek)
- Update README with callbacks documentation

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

media_server/routes/__init__.py

@@ -1,7 +1,8 @@
 """API route modules."""
 
+from .audio import router as audio_router
 from .health import router as health_router
 from .media import router as media_router
 from .scripts import router as scripts_router
 
-__all__ = ["health_router", "media_router", "scripts_router"]
+__all__ = ["audio_router", "health_router", "media_router", "scripts_router"]