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/config/config_test.go

@@ -199,6 +199,7 @@ func clearConfigEnv(t *testing.T) {
 		"WARPBOX_THUMBNAIL_BATCH_SIZE",
 		"WARPBOX_THUMBNAIL_INTERVAL_SECONDS",
 		"WARPBOX_SECURITY_ENABLED",
+		"WARPBOX_EXPIRED_CLEANUP_INTERVAL_SECONDS",
 	} {
 		t.Setenv(name, "")
 	}