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>

web/src/admin.ts

@@ -34,11 +34,11 @@ async function drawServer(){
       </div>
       <span class="hint">Applies to every feed that does not set its own. A feed's suggested
       interval (its <b>ttl</b>) is still honoured when it asks to be polled less often.</span></div>
-    <div class="field"><label>Max new downloads per scan, per feed</label>
+    <div class="field"><label>Newest episodes to download, per feed</label>
       <input type="number" id="gmax" min="0" max="999" value="${g.max_new_per_check}">
       <span class="hint">Applies to any feed that does not set its own — including every feed
-      inside an OPML subscription. <b>0 means unlimited</b>, which will pull a whole back
-      catalogue the first time a feed is scanned.</span></div>
+      inside an OPML subscription. Older episodes stay listed to download by hand. <b>0 means
+      every episode</b>, the whole back catalogue.</span></div>
     <div class="field"><label>Download these media types automatically</label>
       <input type="text" id="gtypes" value="${esc((g.media_types||[]).join(', '))}" placeholder="audio, video">
       <span class="hint">Anything else is still listed and can be downloaded by hand — blog feeds