feat(router): add system settings routes and handler wiring

Router configuration updates:
- Add SystemSettingsHandler to Config struct
- Register GET /api/libraries/scan-settings (admin-only)
- Register PUT /api/libraries/scan-settings (admin-only)
- Wire SystemSettingsHandler in main.go

These routes replace per-user scan settings endpoints with
system-wide admin-only endpoints.

internal/router/library.go

@@ -49,6 +49,10 @@ func registerLibraryRoutes(cfg *Config) {
 		return cfg.MediaHandler.ListMediaItems(c)
 	})
 
+	// System scan settings (admin-only)
+	adminLibrary.GET("/scan-settings", cfg.SystemSettingsHandler.GetScanSettings)
+	adminLibrary.PUT("/scan-settings", cfg.SystemSettingsHandler.UpdateScanSettings)
+
 	// User library visibility control
 	userLibrary := library.Group("/visibility")
 	userLibrary.GET("", cfg.LibraryHandler.GetUserVisibleLibraries)