feat(playlists): act on the selected rows from the toolbar, and undo either

While rows are selected the toolbar's second row is the band: the count,
the last target as one click, Move to, Remove, Clear selection. Both write
in one request and undo back to the places; a video the target held before
stays there. Delete takes the selection when there is one. A failure puts
the rows back ticked.

A row's own move goes through the same write, so it undoes too, and the
single-row move route and service function are gone. A playlist row drops
its grip below a 24rem table: the row menu keeps the moves.

backend/tests/test_playlist_position_lock.py

@@ -107,7 +107,8 @@ PLEX = PlexPlaylist.__table__.name
         ),
         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/v1/move", json={"target_playlist_id": other}
+            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"]}),
     ],
@@ -173,7 +174,8 @@ PLEX_ITEMS = PlexPlaylistItem.__table__.name
         ),
         lambda c, pid, other: c.delete(f"/api/playlists/{pid}/items/v1"),
         lambda c, pid, other: c.post(
-            f"/api/playlists/{pid}/items/v1/move", json={"target_playlist_id": other}
+            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"]}),
         # the ORM cascade deletes the items before the list row
@@ -199,7 +201,8 @@ def test_a_move_locks_both_lists_in_one_ordered_statement(db, make_user, as_user
     pid, other = _yt(db, user)
     statements.clear()
     r = as_user(user).post(
-        f"/api/playlists/{pid}/items/v1/move", json={"target_playlist_id": other}
+        f"/api/playlists/{pid}/items/move-bulk",
+        json={"video_ids": ["v1"], "target_playlist_id": other},
     )
     assert r.status_code == 200, r.text
     lock = _locked_before_items(statements, YT, YT_ITEMS)