get_entries(sort='recent') heuristic does not work with backdated entries

[lemon24]

As of 2.12, the recent entry sort heuristic is defined as such:

Most recent first. Currently, that means:

• by import date [added] for entries published less than 7 days ago
• by published date otherwise (if an entry does not have published, updated is used)

This is to make sure newly imported entries appear at the top regardless of when the feed says they were published (sometimes, it lies by a day or two).

For some feeds, entries can appear in the feed months after their published. Example: https://peps.python.org/peps.rss

It should be possible to opt in to see entries when they are added, regardless of published/updated.

Some avenues for doing this:

• Always sort by added, and find a different way of dealing with the order of newly-imported entries.
  • Pro: Might simplify sorting (and make it faster, maybe).
  • Pro: No opt-in needed (no API is better API).
• Use a different recent_threshold for some feeds.
  • Con: Will likely break the current query + indexes.