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/static/js/websocket.js

@@ -6,7 +6,7 @@ import {
     dom, t, showToast, setWs,
     WS_BACKOFF_BASE_MS, WS_BACKOFF_MAX_MS,
     WS_MAX_RECONNECT_ATTEMPTS, WS_PING_INTERVAL_MS,
-    authRequired,
+    authRequired, showUpdateBanner,
 } from './core.js';
 import { updateUI, visualizerEnabled, visualizerAvailable, setFrequencyData, stopPositionInterpolation, loadAudioDevices } from './player.js';
 import { loadScripts, loadScriptsTable, displayQuickAccess } from './scripts.js';
@@ -100,6 +100,8 @@ export function connectWebSocket(token) {
             loadHeaderLinks();
             loadLinksTable();
             displayQuickAccess();
+        } else if (msg.type === 'update_available') {
+            showUpdateBanner(msg.data);
         } else if (msg.type === 'audio_data') {
             setFrequencyData(msg.data);
         } else if (msg.type === 'error') {