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.

frontend/src/lib/api/playlists.ts

@@ -77,16 +77,58 @@ export const playlistsApi = {
     }),
   removeFromPlaylist: (id: number, videoId: string) =>
     req(`/api/playlists/${id}/items/${videoId}`, { method: "DELETE" }),
-  // One request on purpose: add-then-remove from the client leaves the video in BOTH lists when
-  // the second call fails. The server does it in one transaction.
-  moveToPlaylist: (
+  // One request on purpose: add-then-remove from the client leaves a video in BOTH lists when the
+  // second call fails. The server does it in one transaction, and says which landed in the target
+  // and which it already held: the Undo needs to tell them apart.
+  moveManyToPlaylist: (
     id: number,
-    videoId: string,
+    videoIds: string[],
     targetPlaylistId: number,
-  ): Promise<{ ok: true; from_playlist_id: number; to_playlist_id: number; video_id: string }> =>
-    req(`/api/playlists/${id}/items/${videoId}/move`, {
+  ): Promise<{
+    ok: true;
+    from_playlist_id: number;
+    to_playlist_id: number;
+    moved: string[];
+    already_there: string[];
+  }> =>
+    req(`/api/playlists/${id}/items/move-bulk`, {
       method: "POST",
-      body: JSON.stringify({ target_playlist_id: targetPlaylistId }),
+      body: JSON.stringify({ video_ids: videoIds, target_playlist_id: targetPlaylistId }),
+    }),
+  // The Undo of a move, back into `id`. `positions` pairs with `moved` + `alreadyThere`.
+  undoPlaylistMove: (
+    id: number,
+    fromPlaylistId: number,
+    moved: string[],
+    alreadyThere: string[],
+    positions: number[],
+  ): Promise<{ ok: true; playlist_id: number; added: number }> =>
+    req(`/api/playlists/${id}/items/undo-move`, {
+      method: "POST",
+      body: JSON.stringify({
+        from_playlist_id: fromPlaylistId,
+        moved,
+        already_there: alreadyThere,
+        positions,
+      }),
+    }),
+  removeManyFromPlaylist: (
+    id: number,
+    videoIds: string[],
+  ): Promise<{ ok: true; playlist_id: number; removed: number }> =>
+    req(`/api/playlists/${id}/items/remove-bulk`, {
+      method: "POST",
+      body: JSON.stringify({ video_ids: videoIds }),
+    }),
+  // The Undo of a removal, in one request; an index in the stored order per video.
+  restoreToPlaylist: (
+    id: number,
+    videoIds: string[],
+    positions: number[],
+  ): Promise<{ ok: true; playlist_id: number; added: number }> =>
+    req(`/api/playlists/${id}/items/restore`, {
+      method: "POST",
+      body: JSON.stringify({ video_ids: videoIds, positions }),
     }),
   reorderPlaylist: (id: number, videoIds: string[]) =>
     req(`/api/playlists/${id}/order`, {