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/provider-concept.md

@@ -0,0 +1,11 @@
+# Provider Concept
+
+Providers implement these interfaces:
+
+- `HypervisorProvider`
+- `InventoryProvider`
+- `NetworkProvider`
+- `FirewallProvider`
+
+The Proxmox provider is the first implementation. Future providers can be registered without changing API route code.
+