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_test.go

@@ -279,6 +279,7 @@ func clearAdminSettingsEnv(t *testing.T) {
 		"WARPBOX_SECURITY_UPLOAD_MAX_GB",
 		"WARPBOX_SECURITY_UPLOAD_MAX_MB",
 		"WARPBOX_SECURITY_UPLOAD_MAX_BYTES",
+		"WARPBOX_EXPIRED_CLEANUP_INTERVAL_SECONDS",
 	} {
 		t.Setenv(name, "")
 	}