fix: keep mesh/cluster links so edges resolve onto a second canvas

Approving the same zigbee/zwave/proxmox devices onto a second design
placed the nodes but drew no edges. _resolve_pending_links_for_ieee
deleted each pending_device_link after materializing its edge, so the
first approve consumed the whole topology and later approves had nothing
to resolve.

Links are topology, not one-shot: every importer wipes+reinserts its
link set on each import, so they can safely persist across approvals.

- keep the link rows (drop both db.delete(link) calls)
- scope resolution to the target design (Node.ieee_address + design_id)
  so a re-approve links that canvas's nodes, not another's
- thread design_id through all three approve call sites

Existing links deleted by the old code do not come back on their own;
a re-import repopulates them.

ha-relevant: maybe

backend/tests/test_proxmox_router.py

@@ -389,7 +389,7 @@ async def test_cluster_link_resolves_to_cluster_edge(db_session) -> None:
     ))
     await db_session.commit()
 
-    await _resolve_pending_links_for_ieee(db_session, "pve-node-a")
+    await _resolve_pending_links_for_ieee(db_session, "pve-node-a", design.id)
 
     edge = (await db_session.execute(select(Edge))).scalars().one()
     assert edge.type == "cluster"
@@ -399,6 +399,42 @@ async def test_cluster_link_resolves_to_cluster_edge(db_session) -> None:
     assert edge.target_handle == "left"
 
 
+@pytest.mark.asyncio
+async def test_link_survives_and_resolves_onto_second_design(db_session) -> None:
+    # Regression: approving the same mesh devices onto a second canvas must also
+    # get their edges. The link row is topology, not one-shot — resolving it on
+    # design A must not consume it, so design B resolves too.
+    da = Design(id=str(uuid.uuid4()), name="a")
+    db_ = Design(id=str(uuid.uuid4()), name="b")
+    db_session.add_all([da, db_])
+    # Same two devices placed on BOTH designs (one Node per canvas).
+    for d in (da, db_):
+        db_session.add_all([
+            Node(id=str(uuid.uuid4()), type="iot", label="x", ieee_address="0xAAAA",
+                 status="online", pos_x=0, pos_y=0, design_id=d.id),
+            Node(id=str(uuid.uuid4()), type="iot", label="y", ieee_address="0xBBBB",
+                 status="online", pos_x=0, pos_y=0, design_id=d.id),
+        ])
+    db_session.add(PendingDeviceLink(
+        id=str(uuid.uuid4()), source_ieee="0xAAAA", target_ieee="0xBBBB",
+        discovery_source="zigbee",
+    ))
+    await db_session.commit()
+
+    first = await _resolve_pending_links_for_ieee(db_session, "0xAAAA", da.id)
+    second = await _resolve_pending_links_for_ieee(db_session, "0xAAAA", db_.id)
+
+    assert len(first) == 1
+    assert len(second) == 1  # would be 0 if the link were consumed on design A
+    # One iot edge per design, each between that design's own nodes.
+    edges = (await db_session.execute(select(Edge))).scalars().all()
+    assert len(edges) == 2
+    assert {e.design_id for e in edges} == {da.id, db_.id}
+    assert all(e.type == "iot" for e in edges)
+    # The link row is still present for any further design.
+    assert (await db_session.execute(select(PendingDeviceLink))).scalars().one()
+
+
 @pytest.mark.asyncio
 async def test_persist_never_deletes(db_session) -> None:
     await _persist_pending_import(db_session, [_guest_node(101, "10.0.0.5")], [])

backend/tests/test_scan.py

@@ -1375,7 +1375,9 @@ async def test_approve_zigbee_skips_when_other_endpoint_still_pending(
 async def test_approve_zigbee_resolves_link_after_second_approval(
     client: AsyncClient, headers, db_session
 ):
-    """First approval keeps link; second approval creates the edge."""
+    """First approval keeps link (other endpoint pending); second approval
+    creates the edge. The link row is retained afterwards so the same pair can
+    be re-approved onto another canvas — it's topology, wiped only on reimport."""
     from sqlalchemy import select
 
     from app.db.models import Edge, PendingDevice, PendingDeviceLink
@@ -1408,7 +1410,7 @@ async def test_approve_zigbee_resolves_link_after_second_approval(
     edges = (await db_session.execute(select(Edge))).scalars().all()
     assert len(edges) == 1
     links = (await db_session.execute(select(PendingDeviceLink))).scalars().all()
-    assert links == []  # consumed
+    assert len(links) == 1  # retained for re-approval onto other canvases
 
 
 # --- Deep scan: trigger overrides + config persistence ---