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>

tests/ui/fixtures/serve.js

@@ -12,7 +12,10 @@ http.createServer((req, res) => {
   fs.readFile(file, (err, body) => {
     if (err) { res.writeHead(404).end('no'); return; }
     const type = file.endsWith('.mp3') ? 'audio/mpeg'
-      : file.endsWith('.opml') ? 'text/x-opml' : 'application/xml';
+      : file.endsWith('.opml') ? 'text/x-opml'
+      // Artwork as an image, or /api/art refuses it as not one.
+      : file.endsWith('.jpg') ? 'image/jpeg'
+      : file.endsWith('.html') ? 'text/html' : 'application/xml';
     res.writeHead(200, { 'content-type': type, 'content-length': body.length });
     res.end(body);
   });