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/architecture.md

@@ -0,0 +1,20 @@
+# Architecture
+
+NexaFabric is split into five layers:
+
+1. Frontend: React application for daily network, IPAM, policy, and audit operations.
+2. API: FastAPI REST surface with OpenAPI docs, JWT auth, RBAC hooks, and validation.
+3. Domain services: provider registry, policy engine, firewall orchestrator, audit service, and job coordination.
+4. Persistence: PostgreSQL through SQLAlchemy models. Alembic is intended for production migrations.
+5. Workers: background execution for sync, compile, drift detection, IPAM scans, cleanup, and backup export.
+
+Provider interfaces are intentionally separated into `HypervisorProvider`, `InventoryProvider`, `NetworkProvider`, and `FirewallProvider`. The first implementation is Proxmox, but the API layer is not tied directly to Proxmox-specific code.
+
+Firewall orchestration follows this flow:
+
+1. Policy definition is compiled.
+2. Conflicts and broad access warnings are calculated.
+3. Provider-specific preview output is produced.
+4. Audit log records the preview.
+5. A later apply path must verify cluster write mode, acquire a lock, and write a second audit record.
+