feat(zigbee): import as background scan run

Mirrors IP scan flow: POST /zigbee/import-pending now creates a
ScanRun(kind=zigbee, status=running) and returns immediately.
Networkmap fetch + pending upsert run in the background, status
transitions to done/error when finished.

Frontend: import modal closes on submit, scan history shows the
run with a ZIG/IP kind chip and toasts on completion. Pending
modal auto-refreshes when run finishes.

scan_runs.kind column added (default 'ip', idempotent migration).
Existing zigbee tests refactored to exercise _persist_pending_import
directly (background tasks don't see the test session); route test
verifies the run is created with kind=zigbee.

frontend/src/api/client.ts

@@ -125,11 +125,13 @@ export const zigbeeApi = {
     mqtt_tls_insecure?: boolean
   }) =>
     api.post<{
-      pending_created: number
-      pending_updated: number
-      coordinator: { id: string; label: string; ieee_address: string } | null
-      coordinator_already_existed: boolean
-      links_recorded: number
-      device_count: number
+      id: string
+      status: string
+      kind: string
+      ranges: string[]
+      devices_found: number
+      started_at: string
+      finished_at: string | null
+      error: string | null
     }>('/zigbee/import-pending', data),
 }