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/locales/en.json

@@ -224,5 +224,7 @@
   "links.confirm.delete": "Are you sure you want to delete the link \"{name}\"?",
   "links.confirm.unsaved": "You have unsaved changes. Are you sure you want to discard them?",
   "footer.created_by": "Created by",
-  "footer.source_code": "Source Code"
+  "footer.source_code": "Source Code",
+  "update.available": "Update available: v{version}",
+  "update.view_release": "View Release"
 }