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

agent/README.md

@@ -0,0 +1,14 @@
+# NexaFabric Node Agent
+
+The node agent is optional. NexaFabric works through the Proxmox API without it.
+
+Planned agent responsibilities:
+
+- Read nftables status.
+- Report node network health.
+- Apply explicitly approved rule bundles.
+- Authenticate with token or mTLS.
+- Expose a health check.
+
+The current repository includes the service file and install script skeleton so packaging can be added without changing the control-plane architecture.
+

agent/nexafabric_agent.py

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+from http.server import BaseHTTPRequestHandler, HTTPServer
+import json
+import os
+
+
+class HealthHandler(BaseHTTPRequestHandler):
+    def do_GET(self) -> None:
+        if self.path != "/healthz":
+            self.send_response(404)
+            self.end_headers()
+            return
+        body = json.dumps({"status": "ok", "agent": "nexafabric"}).encode()
+        self.send_response(200)
+        self.send_header("Content-Type", "application/json")
+        self.send_header("Content-Length", str(len(body)))
+        self.end_headers()
+        self.wfile.write(body)
+
+
+def main() -> None:
+    host = os.getenv("NEXAFABRIC_AGENT_HOST", "0.0.0.0")
+    port = int(os.getenv("NEXAFABRIC_AGENT_PORT", "9844"))
+    HTTPServer((host, port), HealthHandler).serve_forever()
+
+
+if __name__ == "__main__":
+    main()
+