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/services/release_provider.py

@@ -0,0 +1,29 @@
+"""Abstract release provider interface for version checking."""
+
+from dataclasses import dataclass
+from typing import Protocol
+
+
+@dataclass(frozen=True)
+class ReleaseInfo:
+    """Version-provider-agnostic release metadata."""
+
+    version: str  # e.g. "1.1.0" (no "v" prefix)
+    url: str  # release page URL
+    prerelease: bool
+
+
+class ReleaseProvider(Protocol):
+    """Abstract interface for fetching the latest release.
+
+    Implement this protocol to support different hosting platforms
+    (Gitea, GitHub, GitLab, etc.).
+    """
+
+    async def get_latest_release(self) -> ReleaseInfo | None:
+        """Fetch the latest stable release.
+
+        Returns:
+            ReleaseInfo if a release was found, None on failure.
+        """
+        ...