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

backend/app/core/scheduler.py

@@ -0,0 +1,66 @@
+"""APScheduler setup for background scan and status check jobs."""
+import logging
+from datetime import UTC, datetime
+
+import yaml
+from apscheduler.schedulers.asyncio import AsyncIOScheduler
+from sqlalchemy import select
+
+from app.core.config import settings
+from app.db.database import AsyncSessionLocal
+from app.db.models import Node
+from app.services.status_checker import check_node
+
+logger = logging.getLogger(__name__)
+
+scheduler = AsyncIOScheduler()
+
+
+async def _run_status_checks() -> None:
+    """Check all nodes and broadcast results via WebSocket."""
+    from app.api.routes.status import broadcast_status  # avoid circular import
+
+    async with AsyncSessionLocal() as db:
+        result = await db.execute(select(Node))
+        nodes = result.scalars().all()
+
+    for node in nodes:
+        if not node.check_method:
+            continue
+        try:
+            result = await check_node(node.check_method, node.check_target, node.ip)
+            async with AsyncSessionLocal() as db:
+                n = await db.get(Node, node.id)
+                if n:
+                    n.status = result["status"]
+                    n.response_time_ms = result["response_time_ms"]
+                    n.last_seen = datetime.now(UTC) if result["status"] == "online" else n.last_seen
+                    await db.commit()
+            await broadcast_status(
+                node_id=node.id,
+                status=result["status"],
+                checked_at=datetime.now(UTC).isoformat(),
+                response_time_ms=result["response_time_ms"],
+            )
+        except Exception as exc:
+            logger.error("Status check failed for node %s: %s", node.id, exc)
+
+
+def _load_interval() -> int:
+    try:
+        with open(settings.config_path) as f:
+            cfg = yaml.safe_load(f)
+        return int(cfg.get("status_checker", {}).get("interval_seconds", 60))
+    except Exception:
+        return 60
+
+
+def start_scheduler() -> None:
+    interval = _load_interval()
+    scheduler.add_job(_run_status_checks, "interval", seconds=interval, id="status_checks")
+    scheduler.start()
+    logger.info("Scheduler started — status checks every %ds", interval)
+
+
+def stop_scheduler() -> None:
+    scheduler.shutdown(wait=False)