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/gitea_release_provider.py

@@ -0,0 +1,77 @@
+"""Gitea release provider implementation."""
+
+import json
+import logging
+import urllib.error
+import urllib.request
+from typing import Optional
+
+from .release_provider import ReleaseInfo, ReleaseProvider
+
+logger = logging.getLogger(__name__)
+
+# Default repository coordinates
+_DEFAULT_BASE_URL = "https://git.dolgolyov-family.by"
+_DEFAULT_OWNER = "alexei.dolgolyov"
+_DEFAULT_REPO = "media-player-server"
+
+
+class GiteaReleaseProvider(ReleaseProvider):
+    """Fetches the latest release from a Gitea repository."""
+
+    def __init__(
+        self,
+        base_url: str = _DEFAULT_BASE_URL,
+        owner: str = _DEFAULT_OWNER,
+        repo: str = _DEFAULT_REPO,
+        timeout: float = 10.0,
+    ) -> None:
+        self._api_url = f"{base_url}/api/v1/repos/{owner}/{repo}/releases"
+        self._release_page_url = f"{base_url}/{owner}/{repo}/releases/tag"
+        self._timeout = timeout
+
+    async def get_latest_release(self) -> Optional[ReleaseInfo]:
+        """Fetch the latest stable release from Gitea API.
+
+        Returns:
+            ReleaseInfo for the latest non-prerelease, or None on failure.
+        """
+        import asyncio
+
+        try:
+            data = await asyncio.to_thread(self._fetch_releases)
+        except Exception as e:
+            logger.warning("Failed to check for updates: %s", e)
+            return None
+
+        if not data:
+            return None
+
+        # Find the first non-prerelease, non-draft release
+        for release in data:
+            if release.get("draft") or release.get("prerelease"):
+                continue
+
+            tag = release.get("tag_name", "")
+            version = tag.lstrip("v")
+            if not version:
+                continue
+
+            return ReleaseInfo(
+                version=version,
+                url=f"{self._release_page_url}/{tag}",
+                prerelease=False,
+            )
+
+        logger.debug("No stable releases found")
+        return None
+
+    def _fetch_releases(self) -> list[dict]:
+        """Synchronous HTTP fetch of releases (run in thread)."""
+        url = f"{self._api_url}?limit=5"
+        req = urllib.request.Request(url, headers={"Accept": "application/json"})
+        try:
+            with urllib.request.urlopen(req, timeout=self._timeout) as resp:
+                return json.loads(resp.read().decode())
+        except (urllib.error.URLError, urllib.error.HTTPError, json.JSONDecodeError, OSError) as e:
+            raise RuntimeError(f"Gitea API request failed: {e}") from e