fix: cluster edges render on left/right handles from approve flow

Cluster edges created via the pending -> approve path rendered on the top
handle instead of left/right, because the edge and its endpoints lost their
handle information on the way to the canvas.

- Approve resolver (scan.py) now returns each edge's type + source/target
  handle. Handle IDs are the bare slot-0 side names ('right'/'left'), the
  canonical stored form React Flow resolves to the correct side; a '-t' target
  id fails to resolve and falls back to the top handle.
- Frontend injectAutoEdges no longer hardcodes iot/bottom/top-t. It injects
  each edge with its real type + handles and bumps the referenced nodes'
  left/right handle counts (which default to 0) so the cluster endpoints exist.
  Logic extracted to a pure, tested util (applyAutoEdges).
- clusterEdges direct-import path uses the bare 'left' target to match.

Tests: new autoEdges unit tests; updated backend handle assertions.

ha-relevant: maybe

backend/tests/test_scan.py

@@ -1281,7 +1281,8 @@ async def test_approve_zigbee_creates_edge_when_other_endpoint_is_node(
     assert edges[0].source == coord.id
     assert edges[0].target == data["node_id"]
     assert edges[0].source_handle == "bottom"
-    assert edges[0].target_handle == "top-t"
+    # Bare side name (canonical stored form); renders at the top like before.
+    assert edges[0].target_handle == "top"
     assert edges[0].type == "iot"