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/core.js

@@ -318,12 +318,35 @@ export async function fetchVersion() {
             if (data.version) {
                 label.textContent = `v${data.version}`;
             }
+            if (data.update_available) {
+                showUpdateBanner(data.update_available);
+            }
         }
     } catch (error) {
         console.error('Error fetching version:', error);
     }
 }
 
+export function showUpdateBanner(update) {
+    const dismissed = sessionStorage.getItem('update_dismissed');
+    if (dismissed === update.latest) return;
+
+    const banner = document.getElementById('updateBanner');
+    const text = document.getElementById('updateBannerText');
+    const link = document.getElementById('updateBannerLink');
+    const closeBtn = document.getElementById('updateBannerClose');
+
+    text.textContent = t('update.available', { version: update.latest });
+    link.href = update.url;
+    link.textContent = t('update.view_release');
+    banner.classList.remove('hidden');
+
+    closeBtn.onclick = () => {
+        banner.classList.add('hidden');
+        sessionStorage.setItem('update_dismissed', update.latest);
+    };
+}
+
 // ============================================================
 // Shared Utilities
 // ============================================================