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>

README.md

@@ -17,8 +17,8 @@ behind iPodderX (2004-2008, Ray Slakinski & August Trometer).
   sign in with a password or through a proxy (Cloudflare Zero Trust or Authentik), and admins
   manage accounts and settings.
 - **Scanning.** Feeds are checked on a schedule, globally or per feed, and a feed's own TTL is
-  honoured. Keyword, explicit-content and media-type filters decide what is downloaded, with a cap
-  on new downloads per scan.
+  honoured. Keyword, explicit-content and media-type filters decide what is downloaded, with a limit
+  on how many of a feed's newest episodes are downloaded.
 - **Downloads.** Files come over HTTP or BitTorrent and are filed into a folder per feed.
   Retention deletes the oldest files to stay under a disk quota or an age limit, and never touches
   an item someone has pinned.