feat: Spring Boot backend with deploy pipeline

- REST API: POST/GET/DELETE /api/apps
- Build pipeline: JAR → Dockerfile → docker build → push → kubectl apply
- In-memory state with K8s reconciliation on startup
- Shell executor for docker/kubectl commands
- Configurable via env vars (server URL, bootstrap token, registry, namespace)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

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