Bookmark Episode Sort doesn't sort by Timestamp

### Description

When sorting bookmarks on the Podcast page by "Episode", I expect the bookmarks from a single Episode to be sorted by the Timestamp within that Episode. This is not the case, they get sorted by creation date.

### Step-by-step reproduction instructions

1. Create multiple bookmarks within single Episode, preferably creating ones later in the Episode first.
2. Go to Podcast page
3. Go to Bookmarks tab
4. Sort by "Episode"
5. Notice that bookmark within a single episode get sorted by creation time, not Timestamp within Episode

### Screenshots or screen recording

_No response_

### Did you search for existing bug reports?

- [X] I have searched for existing bug reports.

### Device, Operating system, and Pocket Casts app version

_No response_

Reproduce the issue using the listed steps: create bookmarks later in an episode first, then open the Podcast page's Bookmarks tab and sort by Episode. Trace the bookmark sorting entry point and verify that bookmarks sharing an episode are ordered by their episode timestamp rather than creation date; done means the reproduction shows timestamp order.

Written by the indexing model from the issue text.