feat: add update-available notification system

- Abstract ReleaseProvider protocol for platform-agnostic version checking
- GiteaReleaseProvider implementation using stdlib urllib
- UpdateChecker service with periodic background checks and WS broadcast
- Persistent dismissible banner in Web UI when a new version is detected
- Health endpoint now returns cached update info
- Configurable via update_check_enabled and update_check_interval settings
- i18n support (EN/RU)

media_server/main.py

@@ -74,6 +74,18 @@ async def lifespan(app: FastAPI):
     await ws_manager.start_status_monitor(controller.get_status)
     logger.info("WebSocket status monitor started")
 
+    # Start update checker
+    update_checker = None
+    if settings.update_check_enabled:
+        from .services.gitea_release_provider import GiteaReleaseProvider
+        from .services.update_checker import UpdateChecker
+
+        provider = GiteaReleaseProvider()
+        update_checker = UpdateChecker(provider, __version__)
+        await update_checker.start(settings.update_check_interval)
+        # Store globally so health endpoint can access cached result
+        app.state.update_checker = update_checker
+
     # Register audio visualizer (capture starts on-demand when clients subscribe)
     analyzer = None
     if settings.visualizer_enabled:
@@ -92,6 +104,10 @@ async def lifespan(app: FastAPI):
 
     yield
 
+    # Stop update checker
+    if update_checker is not None:
+        await update_checker.stop()
+
     # Stop audio visualizer
     await ws_manager.stop_audio_monitor()
     if analyzer and analyzer.running: