Add WebSocket support for real-time media status updates

Replace HTTP polling with WebSocket push notifications for instant
state change responses. Server broadcasts updates only when significant
changes occur (state, track, volume, etc.) while letting Home Assistant
interpolate position during playback. Includes seek detection for
timeline updates and automatic fallback to HTTP polling if WebSocket
disconnects.

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

custom_components/remote_media_player/__init__.py

@@ -138,6 +138,12 @@ async def async_unload_entry(hass: HomeAssistant, entry: ConfigEntry) -> bool:
     if unload_ok:
         # Close client and remove data
         data = hass.data[DOMAIN].pop(entry.entry_id)
+
+        # Shutdown coordinator (WebSocket cleanup)
+        if "coordinator" in data:
+            await data["coordinator"].async_shutdown()
+
+        # Close HTTP client
         await data["client"].close()
 
         # Remove services if this was the last entry