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>

docs/configuration.md

@@ -52,8 +52,10 @@ database as described above; `download_dir`, `socket` and `organize` stay in con
   read it. `0` disables it entirely.
 * **`max_age_days`** — items older than this with no file on disk are pruned from the database.
   Kept ones stay. `0` disables it.
-* **`max_new_per_check`** — the cap that stops a new subscription pulling a whole back catalogue.
-  `0` means unlimited, which is rarely what you want: subscribing to an OPML of 80 feeds with no cap
+* **`max_new_per_check`** — how many of a feed's newest episodes are downloaded; older ones stay
+  listed to download by hand. It stops a new subscription pulling a whole back catalogue.
+  `0` means every episode, for an archive; set it on the feeds you want archived, since on the
+  global default it applies to every feed: subscribing to an OPML of 80 feeds with no limit
   fetched 216 files and 22 GB in one scan.
 * **`media_types`** — top-level MIME types taken automatically. Anything else is still listed and
   can be fetched by hand; blog feeds put each article's header image in an `<enclosure>`, and