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

backend/app/services/providers/registry.py

@@ -0,0 +1,10 @@
+from app.services.providers.base import Provider
+from app.services.providers.demo import DemoProvider
+from app.services.providers.proxmox import ProxmoxProvider
+
+
+_providers: dict[str, Provider] = {"proxmox": ProxmoxProvider(), "demo": DemoProvider()}
+
+
+def get_provider(name: str) -> Provider:
+    return _providers[name]