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

templates/templates.go

@@ -45,6 +45,7 @@ func initTemplates() {
 		"app_edit.html",
 		"deployments.html",
 		"webhook_events.html",
+		"backup_import.html",
 	}
 
 	pageTemplates = make(map[string]*template.Template)