fix(meshcore): revert wrong scan_ble_sync route call, fix scan without worker

- Revert route to scan_ble() — scan_ble_sync() lives on AsyncWorker,
  not MeshcoreClient; the 500 was caused by our previous fix
- MeshcoreClient.scan_ble() now runs a one-shot asyncio scan when no
  worker is active, so Scan works before Connect is pressed

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

routes/meshcore.py

@@ -93,7 +93,7 @@ def ports():
 def ble_scan():
     if not is_meshcore_available():
         return api_error("meshcore not installed", 503)
-    devices = _client().scan_ble_sync()
+    devices = _client().scan_ble()
     return jsonify({"devices": devices})