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/util.ts

@@ -145,10 +145,11 @@ const tint=name=>{ let h=0; for(const c of name||'?') h=(h*31+c.charCodeAt(0))>>
 function tileHTML(name,cls){
   return `<div class="art ini ${cls||''}" style="--tint:${tint(name)}">${esc(initials(name))}</div>`;
 }
-/// On the https page, the browser upgrades an http:// image to https, and a host that has no
-/// https shows nothing (issue #90); ipx fetches those itself.
+/// Every image a feed names comes from iPX, which keeps a copy: fast after the first time, and
+/// no publisher's server asked on every visit. It began with http-only artwork, which the https
+/// page could not load (#90).
 function artSrc(url: string){
-  return location.protocol==='https:'&&/^http:\/\//i.test(url) ? '/api/art?u='+encodeURIComponent(url) : url;
+  return /^https?:\/\//i.test(url) ? '/api/art?u='+encodeURIComponent(url) : url;
 }
 function artHTML(url: string | null, name: string, cls?: string){
   return url