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/routes/health.py

@@ -3,23 +3,31 @@
 import platform
 from typing import Any
 
-from fastapi import APIRouter
+from fastapi import APIRouter, Request
 
+from .. import __version__
 from ..auth import auth_enabled
 
 router = APIRouter(prefix="/api", tags=["health"])
 
 
 @router.get("/health")
-async def health_check() -> dict[str, Any]:
+async def health_check(request: Request) -> dict[str, Any]:
     """Health check endpoint - no authentication required.
 
     Returns:
         Health status and server information
     """
-    return {
+    result: dict[str, Any] = {
         "status": "healthy",
         "platform": platform.system(),
-        "version": "1.0.0",
+        "version": __version__,
         "auth_required": auth_enabled(),
     }
+
+    # Include cached update info if available
+    checker = getattr(request.app.state, "update_checker", None)
+    if checker is not None and checker.cached_update is not None:
+        result["update_available"] = checker.cached_update
+
+    return result