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/css/styles.css

@@ -3490,6 +3490,59 @@ footer .separator {
     }
 }
 
+/* Update Banner */
+.update-banner {
+    position: fixed;
+    top: 0;
+    left: 0;
+    right: 0;
+    z-index: 1001;
+    display: flex;
+    align-items: center;
+    justify-content: center;
+    gap: 12px;
+    padding: 10px 16px;
+    background: var(--accent);
+    color: #fff;
+    font-size: 0.85rem;
+    font-weight: 500;
+    text-align: center;
+    transition: transform 0.3s ease;
+}
+
+.update-banner:not(.hidden) {
+    animation: bannerSlideIn 0.4s ease-out;
+}
+
+.update-banner.hidden {
+    transform: translateY(-100%);
+    pointer-events: none;
+}
+
+.update-banner a {
+    color: #fff;
+    text-decoration: underline;
+    font-weight: 600;
+}
+
+.update-banner a:hover {
+    opacity: 0.85;
+}
+
+.update-banner-close {
+    background: none;
+    color: #fff;
+    font-size: 1.2rem;
+    padding: 0 4px;
+    opacity: 0.7;
+    cursor: pointer;
+    line-height: 1;
+}
+
+.update-banner-close:hover {
+    opacity: 1;
+}
+
 /* Connection Banner */
 .connection-banner {
     position: fixed;