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/demo.py

@@ -0,0 +1,35 @@
+from typing import Any
+
+from app.services.providers.base import Provider, ProviderConnection
+
+
+class DemoProvider(Provider):
+    name = "demo"
+
+    async def test_connection(self, connection: ProviderConnection) -> dict[str, Any]:
+        return {"version": "demo", "api_url": connection.api_url, "mode": "offline"}
+
+    async def sync_inventory(self, connection: ProviderConnection) -> dict[str, list[dict[str, Any]]]:
+        return {
+            "nodes": [
+                {"node": "demo-pve-01", "status": "online", "maxcpu": 16, "maxmem": 68719476736},
+                {"node": "demo-pve-02", "status": "online", "maxcpu": 16, "maxmem": 68719476736},
+            ],
+            "workloads": [
+                {"vmid": 201, "name": "demo-web-01", "type": "qemu", "status": "running", "node": "demo-pve-01"},
+                {"vmid": 202, "name": "demo-db-01", "type": "qemu", "status": "running", "node": "demo-pve-02"},
+            ],
+            "networks": [
+                {"name": "vmbr0", "type": "bridge", "vlan": 10},
+                {"name": "vnet-prod", "type": "vnet", "vlan": 120},
+            ],
+        }
+
+    async def list_networks(self, connection: ProviderConnection) -> list[dict[str, Any]]:
+        return (await self.sync_inventory(connection))["networks"]
+
+    async def preview_rules(self, connection: ProviderConnection, rules: list[dict[str, Any]]) -> dict[str, Any]:
+        return {"provider": self.name, "generated": rules, "warnings": ["Demo provider preview."]}
+
+    async def apply_rules(self, connection: ProviderConnection, rules: list[dict[str, Any]]) -> dict[str, Any]:
+        return {"applied": False, "reason": "Demo provider never applies firewall changes.", "rules": rules}