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/installation.md

@@ -0,0 +1,25 @@
+# Installation
+
+## Docker Compose
+
+```bash
+cp .env.example .env
+docker compose up --build
+```
+
+Change every secret in `.env` before using NexaFabric outside a local lab.
+
+## Proxmox API Token
+
+Create a least-privilege Proxmox API token and start in read-only mode. NexaFabric can inventory clusters, nodes, VMs, LXCs, networks, and firewall state before any write workflows are enabled.
+
+Recommended initial scope:
+
+- Cluster inventory read
+- Node inventory read
+- VM and LXC config read
+- SDN and network read
+- Firewall read
+
+Enable write permissions only for a dedicated automation token after previews and approvals are working.
+