Pace IGDB requests; rewrite the new help text

IGDB allows 4 requests a second, and an import searching three entries
at once (plus detail lookups) went over it. Every IGDB request now waits
at least 300 ms after the last one, and a 429 is retried after 1, 2 and
4 seconds before giving up.

The import dialog, ranking editor and their error messages are reworded
in plainer language, and talk about "spots" instead of "places".

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

web/templates/pages/admin_list.html

@@ -39,7 +39,7 @@
     <section class="panel editor-items" aria-labelledby="items-h">
       <div class="panel-head">
         <h2 id="items-h">Ranking</h2>
-        <span class="muted small">Drag, or type a place and press Enter. Places can be left empty.</span>
+        <span class="muted small">Drag to reorder, or type a number. Spots can stay open.</span>
       </div>
       <ol class="rank-list" data-rank-list></ol>
     </section>