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

README.md

@@ -11,6 +11,7 @@ A simple self-hosted PaaS that auto-deploys Docker containers from Git repositor
 - Environment variables, labels, and volume mounts per app
 - Docker builds via socket access
 - Notifications via ntfy and Slack-compatible webhooks
+- Backup/restore of app configurations (JSON export/import via UI and API)
 - Simple server-rendered UI with Tailwind CSS
 
 ## Non-Goals