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/config.py

@@ -159,6 +159,17 @@ class Settings(BaseSettings):
         description="Loopback audio device name for visualizer (None = auto-detect)",
     )
 
+    # Update checker
+    update_check_enabled: bool = Field(
+        default=True,
+        description="Check for new versions on startup and periodically",
+    )
+    update_check_interval: int = Field(
+        default=21600,
+        description="Update check interval in seconds (default: 6 hours)",
+        ge=600,
+    )
+
     @classmethod
     def load_from_yaml(cls, path: Optional[Path] = None) -> "Settings":
         """Load settings from a YAML configuration file."""