feat: add backup/restore of app configurations

Add export and import functionality for app configurations:

- Export single app or all apps as versioned JSON backup bundle
- Import from backup file with name-conflict detection (skip duplicates)
- Fresh SSH keys and webhook secrets generated on import
- Preserves env vars, labels, volumes, and port mappings
- Web UI: export button on app detail, backup/restore page on dashboard
- REST API: GET /api/v1/apps/{id}/export, GET /api/v1/backup/export,
  POST /api/v1/backup/import
- Comprehensive test coverage for service and handler layers

internal/handlers/api_test.go

@@ -27,6 +27,11 @@ func apiRouter(tc *testContext) http.Handler {
 			apiR.Get("/apps", tc.handlers.HandleAPIListApps())
 			apiR.Get("/apps/{id}", tc.handlers.HandleAPIGetApp())
 			apiR.Get("/apps/{id}/deployments", tc.handlers.HandleAPIListDeployments())
+
+			// Backup/Restore API
+			apiR.Get("/apps/{id}/export", tc.handlers.HandleAPIExportApp())
+			apiR.Get("/backup/export", tc.handlers.HandleAPIExportAllApps())
+			apiR.Post("/backup/import", tc.handlers.HandleAPIImportApps())
 		})
 	})