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/developer-guide.md

@@ -0,0 +1,23 @@
+# Developer Guide
+
+Backend:
+
+```bash
+cd backend
+pip install ".[dev]"
+pytest
+ruff check .
+uvicorn app.main:app --reload
+```
+
+Frontend:
+
+```bash
+cd frontend
+npm install
+npm run dev
+npm test
+```
+
+The API docs are served at `/api/docs`.
+