feat: drag-reorder slots

pool.reorder() permutes slots (validated permutation) and keeps the
active selection on its slot; exposed via /grid_pool/reorder. The grid
thumbnails are drag handles; dropping on another cell reorders.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

gates/routes.py

@@ -60,6 +60,13 @@ async def _set_mask(request):
     return web.json_response(m)
 
 
+@routes.post("/grid_pool/reorder")
+async def _reorder(request):
+    body = await request.json()
+    order = [int(i) for i in body["order"]]
+    return web.json_response(handlers.handle_reorder(_base(), body["pool_id"], order))
+
+
 @routes.get("/grid_pool/list")
 async def _list(request):
     pool_id = request.query.get("pool_id", "default")