Keep artwork on disk and serve every image from iPX (#111)

The page loaded artwork from each publisher's server, or through /api/art, fetched every time,
for http-only hosts. Nothing was kept, every visit asked every publisher, and artwork went when
a publisher's server did.

- The page draws every image a feed or item names from /api/art. The first time, iPX fetches
  it (only an address a feed or item names, only an image, up to 5 MB, within the feed timeout)
  and keeps it in art/ beside the database, under a hash of its address with its type beside
  it (src/art.rs). Later it comes from disk, which marks it as used.
- art_cache_mb, a server setting on the admin page, 500 by default, caps what is kept: the
  sweep before each scan drops the least recently shown until it fits. 0 keeps nothing, and
  artwork is fetched through iPX each time. Settings saved before it get the default.
- Served from iPX's own address, someone else's image must stay an image: nosniff, and a CSP
  with sandbox, so an SVG opened on its own runs no script as iPX.
- The fixture server sends .jpg as image/jpeg, which /api/art requires.

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

web/src/admin.ts

@@ -49,6 +49,10 @@ async function drawServer(){
       items are never touched.</span></div>
     <div class="field"><label>Delete items older than (days, 0 = keep)</label>
       <input type="number" id="gage" min="0" value="${g.max_age_days}"></div>
+    <div class="field"><label>Artwork kept (MB, 0 = none)</label>
+      <input type="number" id="gart" min="0" value="${g.art_cache_mb}">
+      <span class="hint">Show and episode artwork is kept here once shown, so it loads from iPX
+      instead of each publisher. Over this, what has gone longest unshown is dropped first.</span></div>
     <div class="field"><label>Download folder</label>
       <span class="hint" style="overflow-wrap:anywhere">${esc(g.download_dir)}</span></div>
     <div class="cardacts"><button class="btn primary" id="gsave">${ICON.check} Save</button></div>`;
@@ -59,7 +63,8 @@ async function drawServer(){
         max_new_per_check: Math.max(0, Number($('#gmax').value) || 0),
         media_types: $('#gtypes').value.split(',').map(t => t.trim()).filter(Boolean),
         max_total_gb: Number($('#gquota').value) || 0,
-        max_age_days: Number($('#gage').value) || 0})});
+        max_age_days: Number($('#gage').value) || 0,
+        art_cache_mb: Math.max(0, Number($('#gart').value) || 0)})});
       toast('Settings saved');
     }catch(e){ toast(e.message, true); }
   };