feat: Phase 3 & 4 — monitoring, discovery, polish, deployment

Phase 3 — Discovery & Monitoring:
- Network scanner: nmap wrapper + mock fallback, fingerprint service (35 signatures)
- Status checker: ping/http/https/tcp/ssh/prometheus/health per-node checks
- APScheduler: status checks every 60s, WebSocket broadcast
- WebSocket /ws/status: live node status updates to frontend
- Sidebar panels: Pending Devices, Hidden Devices, Scan History
- Auth token persisted to localStorage (survive page refresh)
- 24 new backend tests (scan flow + status_checker)

Phase 4 — Polish & Deployment:
- Auto-layout: Dagre hierarchical TB via Toolbar button
- Export PNG: html-to-image download via Toolbar button
- Scan config modal: CIDR ranges + check interval, GET/POST /api/v1/scan/config
- Dockerfile.backend (Python 3.13 slim + nmap), Dockerfile.frontend (nginx)
- docker-compose.yml with data volume and NET_RAW cap for ping
- scripts/lxc-install.sh: Proxmox VE systemd bootstrap
- README.md: quick-start, config reference, stack overview

Dockerfile.backend

@@ -0,0 +1,18 @@
+FROM python:3.13-slim
+
+WORKDIR /app
+
+# Install nmap for network scanning
+RUN apt-get update && apt-get install -y --no-install-recommends nmap && rm -rf /var/lib/apt/lists/*
+
+COPY backend/requirements.txt .
+RUN pip install --no-cache-dir -r requirements.txt
+
+COPY backend/ .
+
+# Create data directory (volume mount point)
+RUN mkdir -p /app/data
+
+EXPOSE 8000
+
+CMD ["uvicorn", "app.main:app", "--host", "0.0.0.0", "--port", "8000"]