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_zwave_router.py

@@ -445,3 +445,38 @@ async def test_persist_keeps_hidden_hidden(db_session) -> None:
         )
     ).scalar_one()
     assert still_hidden.status == "hidden"
+
+
+@pytest.mark.asyncio
+async def test_persist_device_on_multiple_canvases(db_session) -> None:
+    """Regression: a device on TWO designs (one Node each) must not crash
+    re-import with MultipleResultsFound — props refresh on both nodes."""
+    from sqlalchemy import select
+
+    from app.api.routes.zwave import _persist_pending_import
+    from app.db.models import Design, Node
+
+    d1 = Design(name="d1")
+    d2 = Design(name="d2")
+    db_session.add_all([d1, d2])
+    await db_session.flush()
+    for d in (d1, d2):
+        db_session.add(Node(
+            label="Wall Plug", type="zwave_router", status="online",
+            check_method="none", ieee_address="zwave-0xh-2", services=[],
+            properties=[], design_id=d.id,
+        ))
+    await db_session.commit()
+
+    bumped = [dict(n) for n in _PENDING_NODES]
+    bumped[1]["model"] = "ZW200"
+    # Must not raise.
+    await _persist_pending_import(db_session, bumped, _PENDING_EDGES)
+
+    nodes = (
+        await db_session.execute(select(Node).where(Node.ieee_address == "zwave-0xh-2"))
+    ).scalars().all()
+    assert len(nodes) == 2  # both canvas placements preserved
+    for n in nodes:
+        model = {p["key"]: p["value"] for p in n.properties}.get("Model")
+        assert model == "ZW200"  # refreshed on every canvas