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/app/api/routes/zwave.py

@@ -23,6 +23,7 @@ from app.schemas.zwave import (
     ZwaveTestConnectionRequest,
     ZwaveTestConnectionResponse,
 )
+from app.services.node_dedupe import dedupe_nodes_by_ieee
 from app.services.zwave_service import (
     build_zwave_properties,
     fetch_zwave_network,
@@ -134,6 +135,10 @@ async def _persist_pending_import(
     Coordinator auto-approves to a canvas Node. Other devices upsert by Z-Wave
     identity. All zwave-source links are wiped and re-inserted from the new map.
     """
+    # Repair any pre-existing same-canvas duplicate nodes before upserting, so
+    # the by-IEEE lookups below resolve cleanly.
+    await dedupe_nodes_by_ieee(db)
+
     first_design = (await db.execute(select(Design).order_by(Design.created_at).limit(1))).scalar()
     default_design_id = first_design.id if first_design else None
 
@@ -149,15 +154,22 @@ async def _persist_pending_import(
         props = build_zwave_properties(ieee, n.get("vendor"), n.get("model"))
 
         if n.get("type") == "zwave_coordinator":
-            existing = await db.execute(select(Node).where(Node.ieee_address == ieee))
-            existing_node = existing.scalar_one_or_none()
-            if existing_node:
-                existing_node.properties = merge_zwave_properties(
-                    existing_node.properties, props
+            # The coordinator may sit on several canvases (one Node per design).
+            # Refresh props on every matching node, not just one.
+            existing_nodes = (
+                await db.execute(
+                    select(Node).where(Node.ieee_address == ieee).order_by(Node.id)
                 )
+            ).scalars().all()
+            if existing_nodes:
+                for existing_node in existing_nodes:
+                    existing_node.properties = merge_zwave_properties(
+                        existing_node.properties, props
+                    )
+                first = existing_nodes[0]
                 coordinator_out = ZwaveCoordinatorOut(
-                    id=existing_node.id,
-                    label=existing_node.label,
+                    id=first.id,
+                    label=first.label,
                     ieee_address=ieee,
                 )
                 coordinator_existed = True
@@ -180,15 +192,18 @@ async def _persist_pending_import(
             )
             continue
 
-        # Already approved as a canvas Node → refresh props, skip pending row.
-        existing_node_q = await db.execute(
-            select(Node).where(Node.ieee_address == ieee)
-        )
-        existing_node = existing_node_q.scalar_one_or_none()
-        if existing_node:
-            existing_node.properties = merge_zwave_properties(
-                existing_node.properties, props
+        # Already approved as a canvas Node → refresh props on every canvas it
+        # sits on, skip pending row.
+        existing_nodes = (
+            await db.execute(
+                select(Node).where(Node.ieee_address == ieee).order_by(Node.id)
             )
+        ).scalars().all()
+        if existing_nodes:
+            for existing_node in existing_nodes:
+                existing_node.properties = merge_zwave_properties(
+                    existing_node.properties, props
+                )
             continue
 
         result = await db.execute(