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_proxmox_router.py

@@ -265,7 +265,9 @@ async def test_cluster_link_resolves_to_cluster_edge(db_session) -> None:
     edge = (await db_session.execute(select(Edge))).scalars().one()
     assert edge.type == "cluster"
     assert edge.source_handle == "right"
-    assert edge.target_handle == "left-t"
+    # Bare side name (canonical) so React Flow resolves it to the left side;
+    # a '-t' target would fall back to the top handle.
+    assert edge.target_handle == "left"
 
 
 @pytest.mark.asyncio