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>

tests/test_routes_logic.py

@@ -29,3 +29,11 @@ def test_handle_set_mask(tmp_path):
     m = handlers.handle_set_mask(base, "p1", 0, b"MASKBYTES")
     assert m["slots"][0]["mask"] == "img_0001.mask.png"
     assert (tmp_path / "p1" / "img_0001.mask.png").read_bytes() == b"MASKBYTES"
+
+
+def test_handle_reorder(tmp_path):
+    base = str(tmp_path)
+    handlers.handle_add(base, "p1", _png_bytes(), "png", ts=1)
+    handlers.handle_add(base, "p1", _png_bytes(), "png", ts=2)
+    m = handlers.handle_reorder(base, "p1", [1, 0])
+    assert [s["image"] for s in m["slots"]] == ["img_0002.png", "img_0001.png"]