chore: initial project setup with backend, frontend, CI/CD, and documentation

Add complete NexaFabric project structure including:
- FastAPI backend with SQLAlchemy models, JWT auth, RBAC, audit logging, and provider interfaces
- React + TypeScript frontend with Vite, Tailwind CSS, TanStack Query, and Zustand
- Docker Compose configuration for PostgreSQL, Redis, API, worker, frontend, and nginx
- GitHub Actions and GitLab CI workflows for testing, linting, building, and security scanning
- Environment

docs/security.md

@@ -0,0 +1,12 @@
+# Security Concept
+
+- Passwords use Argon2id through Passlib.
+- JWT access tokens are short-lived; refresh token rotation is part of the auth roadmap.
+- API tokens are represented as references in the current scaffold and must be encrypted before production use.
+- RBAC is role and permission based.
+- Firewall changes require preview, validation, locking, and audit records.
+- Proxmox write-enabled mode is explicit per cluster.
+- No dangerous default password should be used in production.
+- CORS origins are configured through environment settings.
+- SQL queries use SQLAlchemy ORM constructs.
+