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

frontend/src/api/client.ts

@@ -88,7 +88,7 @@ export const scanApi = {
       approved: boolean
       node_id: string
       edges_created: number
-      edges: { id: string; source: string; target: string }[]
+      edges: { id: string; source: string; target: string; type?: string; source_handle?: string | null; target_handle?: string | null }[]
     }>(`/scan/pending/${id}/approve`, nodeData),
   hide: (id: string) => api.post(`/scan/pending/${id}/hide`),
   ignore: (id: string) => api.post(`/scan/pending/${id}/ignore`),
@@ -98,7 +98,7 @@ export const scanApi = {
       node_ids: string[]
       device_ids: string[]
       edges_created: number
-      edges: { id: string; source: string; target: string }[]
+      edges: { id: string; source: string; target: string; type?: string; source_handle?: string | null; target_handle?: string | null }[]
       skipped: number
     }>('/scan/pending/bulk-approve', { device_ids: ids, design_id: designId ?? undefined }),
   bulkHide: (ids: string[]) => api.post<{ hidden: number; skipped: number }>('/scan/pending/bulk-hide', { device_ids: ids }),