feat(metrics): add instance_metric and instance_nic_metric models

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/tests/test_metrics_models.py

@@ -0,0 +1,18 @@
+from datetime import datetime, timezone
+
+def test_instance_metric_roundtrip(db_session):
+    from app.models import InstanceMetric, InstanceNicMetric
+    ts = datetime(2026, 6, 21, 12, 0, tzinfo=timezone.utc)
+    db_session.add(InstanceMetric(
+        server_id=1, vmid=114, vm_type="qemu", ts=ts,
+        cpu=0.25, mem=100, maxmem=200, disk_used=10, disk_total=40,
+        netin_rate=1.0, netout_rate=2.0, diskread_rate=0.0, diskwrite_rate=3.0,
+        iops_read=12.0, iops_write=4.0,
+    ))
+    db_session.add(InstanceNicMetric(
+        server_id=1, vmid=114, ts=ts, dev="tap114i0", in_rate=5.0, out_rate=6.0))
+    db_session.commit()
+    m = db_session.query(InstanceMetric).one()
+    assert m.vmid == 114 and m.iops_read == 12.0
+    n = db_session.query(InstanceNicMetric).one()
+    assert n.dev == "tap114i0" and n.out_rate == 6.0