feat(config): add expired box cleanup functionality

Adds dedicated config setting and cleanup logic for
managing expired content boxes via admin tools and CLI.

lib/server/admin_settings.go

@@ -451,6 +451,8 @@ func settingsCategoryForKey(key string) string {
 		return "storage"
 	case config.SettingBoxPollIntervalMS, config.SettingThumbnailBatchSize, config.SettingThumbnailIntervalSeconds:
 		return "workers"
+	case config.SettingExpiredCleanupIntervalSecs:
+		return "workers"
 	default:
 		return "accounts"
 	}
@@ -489,6 +491,7 @@ func settingsDescription(key string) string {
 		config.SettingSecurityUploadWindowSecs:  "Window used for per-IP upload throttling.",
 		config.SettingSecurityUploadMaxRequests: "Max upload requests per IP per upload window.",
 		config.SettingSecurityUploadMaxGB:       "Max upload volume in GB per IP per upload window.",
+		config.SettingExpiredCleanupIntervalSecs: "Background interval for deleting expired boxes. Set 0 to disable periodic cleanup.",
 	}
 	return descriptions[key]
 }