fix(playlists): what the first review of bulk actions found

An Esc during a pointer drag is spent on the drag and leaves the selection.
A wheel over the rail re-reads the target each frame, so a drop lands on the
row under the pointer. After a bulk removal the focus goes to the next row
left, and to the list's menu when the list is emptied.

remove-bulk names the videos that went, and its Undo brings back only those.
An Undo of a move keeps each row (and when it was saved), leaves out a video
that moved on from the target, closes up its place, and says how many came
back. Delete with every tick hidden removes nothing and says why. A list gone
before the lock answers 404, and a body without its videos 422.

One removal path now: the single row goes through the bulk one, the same
toast and undo, which also retires the add-at-position route. railDrop reuses
the store and scrollParent, the drag image the overlay and the z-index token,
and the sensors keep their number. The count is read out from a region that
outlives the band. Tests fill the gaps the review named, each seen failing.

backend/tests/test_playlist_position_lock.py

@@ -102,17 +102,24 @@ PLEX = PlexPlaylist.__table__.name
     "call",
     [
         lambda c, pid, other: c.post(f"/api/playlists/{pid}/items", json={"video_id": "v2"}),
-        lambda c, pid, other: c.post(
-            f"/api/playlists/{pid}/items", json={"video_id": "v2", "position": 0}
-        ),
         lambda c, pid, other: c.post("/api/playlists/watch-later", json={"video_id": "v2"}),
         lambda c, pid, other: c.post(
             f"/api/playlists/{pid}/items/move-bulk",
             json={"video_ids": ["v1"], "target_playlist_id": other},
         ),
         lambda c, pid, other: c.put(f"/api/playlists/{pid}/order", json={"video_ids": ["v1"]}),
+        lambda c, pid, other: c.post(
+            f"/api/playlists/{pid}/items/remove-bulk", json={"video_ids": ["v1"]}
+        ),
+        lambda c, pid, other: c.post(
+            f"/api/playlists/{pid}/items/restore", json={"video_ids": ["v2"], "positions": [0]}
+        ),
+        lambda c, pid, other: c.post(
+            f"/api/playlists/{pid}/items/undo-move",
+            json={"from_playlist_id": other, "moved": ["v2"], "positions": [0]},
+        ),
     ],
-    ids=["add", "add-at", "watch-later", "move", "reorder"],
+    ids=["add", "watch-later", "move", "reorder", "remove-bulk", "restore", "undo-move"],
 )
 def test_a_youtube_playlist_write_locks_the_playlist(db, make_user, as_user, locks, call):
     user = make_user()
@@ -169,9 +176,6 @@ PLEX_ITEMS = PlexPlaylistItem.__table__.name
     "call",
     [
         lambda c, pid, other: c.post(f"/api/playlists/{pid}/items", json={"video_id": "v1"}),
-        lambda c, pid, other: c.post(
-            f"/api/playlists/{pid}/items", json={"video_id": "v2", "position": 0}
-        ),
         lambda c, pid, other: c.delete(f"/api/playlists/{pid}/items/v1"),
         lambda c, pid, other: c.post(
             f"/api/playlists/{pid}/items/move-bulk",
@@ -180,8 +184,27 @@ PLEX_ITEMS = PlexPlaylistItem.__table__.name
         lambda c, pid, other: c.put(f"/api/playlists/{pid}/order", json={"video_ids": ["v1"]}),
         # the ORM cascade deletes the items before the list row
         lambda c, pid, other: c.delete(f"/api/playlists/{pid}"),
+        lambda c, pid, other: c.post(
+            f"/api/playlists/{pid}/items/remove-bulk", json={"video_ids": ["v1"]}
+        ),
+        lambda c, pid, other: c.post(
+            f"/api/playlists/{pid}/items/restore", json={"video_ids": ["v2"], "positions": [0]}
+        ),
+        lambda c, pid, other: c.post(
+            f"/api/playlists/{pid}/items/undo-move",
+            json={"from_playlist_id": other, "moved": ["v2"], "positions": [0]},
+        ),
+    ],
+    ids=[
+        "add-present",
+        "remove",
+        "move",
+        "reorder",
+        "delete",
+        "remove-bulk",
+        "restore",
+        "undo-move",
     ],
-    ids=["add-present", "add-at", "remove", "move", "reorder", "delete"],
 )
 def test_a_youtube_item_write_locks_the_playlist_before_any_item(
     db, make_user, as_user, statements, call
@@ -195,15 +218,23 @@ def test_a_youtube_item_write_locks_the_playlist_before_any_item(
     assert _locked_before_items(statements, YT, YT_ITEMS), statements
 
 
-def test_a_move_locks_both_lists_in_one_ordered_statement(db, make_user, as_user, statements):
-    """Two moves A→B and B→A each locking "its" list first would lock crosswise."""
+@pytest.mark.parametrize(
+    ("path", "body"),
+    [
+        ("move-bulk", lambda other: {"video_ids": ["v1"], "target_playlist_id": other}),
+        ("undo-move", lambda other: {"from_playlist_id": other, "moved": ["v2"], "positions": [0]}),
+    ],
+    ids=["move", "undo"],
+)
+def test_a_move_locks_both_lists_in_one_ordered_statement(
+    db, make_user, as_user, statements, path, body
+):
+    """Two moves A→B and B→A each locking "its" list first would lock crosswise, and an Undo runs the
+    other way round from a move by its nature."""
     user = make_user()
     pid, other = _yt(db, user)
     statements.clear()
-    r = as_user(user).post(
-        f"/api/playlists/{pid}/items/move-bulk",
-        json={"video_ids": ["v1"], "target_playlist_id": other},
-    )
+    r = as_user(user).post(f"/api/playlists/{pid}/items/{path}", json=body(other))
     assert r.status_code == 200, r.text
     lock = _locked_before_items(statements, YT, YT_ITEMS)
     assert lock and "ORDER BY" in lock, lock