fix: tolerate same device on multiple canvases in zigbee/zwave import

Zigbee and Z-Wave imports looked up canvas nodes by ieee_address with
scalar_one_or_none(), assuming one node per IEEE globally. A device placed
on two designs (one Node per canvas — a supported feature) made re-import
crash with MultipleResultsFound.

- Both mesh imports now refresh properties on every matching node instead
  of a single row (loop over .scalars().all()).
- approve_device guards against a true duplicate: same IEEE already on the
  SAME design reuses that node instead of inserting a second one.
- New node_dedupe service: loss-free repair keyed on (ieee, design_id).
  Collapses only genuine same-canvas duplicates (merges properties/services/
  missing fields, re-points edges + parent_id, drops self-loops/parallel
  edges). Cross-design placements preserved. Runs at start of both imports
  and bulk-approve. No-op on healthy DBs.
- IP correlation path already handled multiple nodes; left unchanged.

Tests: dedupe unit tests (collapse, cross-design preservation, edge/parent
re-point, idempotent), zigbee + zwave multi-canvas regression, approve
no-dupe guard.

ha-relevant: yes

backend/tests/test_scan.py

@@ -311,6 +311,44 @@ async def test_approve_device(client: AsyncClient, headers, pending_device):
     assert inventory[0]["status"] == "approved"
 
 
+@pytest.mark.asyncio
+async def test_approve_device_no_dupe_same_ieee_same_design(
+    client: AsyncClient, headers, db_session
+):
+    """Approving a device whose IEEE is already on the target design must reuse
+    the existing node, not create a duplicate (source of the crash bug)."""
+    design = Design(name="d1")
+    db_session.add(design)
+    await db_session.flush()
+    existing = Node(
+        label="sensor", type="zigbee_enddevice", ieee_address="0xZZZ",
+        services=[], design_id=design.id,
+    )
+    db_session.add(existing)
+    device = PendingDevice(
+        id=str(uuid.uuid4()), ieee_address="0xZZZ", suggested_type="zigbee_enddevice",
+        status="pending", discovery_source="zigbee",
+    )
+    db_session.add(device)
+    await db_session.commit()
+
+    res = await client.post(
+        f"/api/v1/scan/pending/{device.id}/approve",
+        json={
+            "label": "sensor", "type": "zigbee_enddevice",
+            "status": "online", "services": [], "design_id": design.id,
+        },
+        headers=headers,
+    )
+    assert res.status_code == 200
+    assert res.json()["node_id"] == existing.id  # reused, not new
+
+    nodes = (
+        await db_session.execute(select(Node).where(Node.ieee_address == "0xZZZ"))
+    ).scalars().all()
+    assert len(nodes) == 1  # no duplicate created
+
+
 @pytest.mark.asyncio
 async def test_approve_nonexistent_device(client: AsyncClient, headers):
     node_payload = {