A download limit means a show's newest episodes, not a pace (#97)

pending() took the newest files still pending, up to the limit, so once a show's latest three
were down, each full read of its feed took the three before them, working back through its
whole history. In production 4420 files (about 310 GB) were queued this way across 12 shows,
all on the default limit of 3, which is meant as "the latest three". It now takes only from the
feed's newest `limit` items with a file. 0, unlimited, still takes the whole back catalogue:
that is how the shows kept as an archive are set, along with limits of 100 and 10000.

The settings' wording followed the old behaviour ("The rest wait for the next scan"); the field
is now "Newest episodes to download", and says what 0 does.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

CHANGELOG.md

@@ -48,6 +48,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Fixed
 
+- A limit on new downloads per check means a show's newest episodes: with it set to 3, ipx no longer works back through the show's history three at a time. Set to 0, it still takes the whole back catalogue.
 - With no limit on new downloads per check (`max_new_per_check = 0`), downloads work on Postgres; they failed.
 - Checking a feed with a long history is much quicker: items already stored are no longer written again on every check.
 - A scan no longer asks a feed's website for its icon every time; it asks again when the artwork changes or you refresh the feed.