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/app.spec.js

@@ -956,6 +956,22 @@ test('reading an item updates its feed\'s count without reloading the list', asy
   expect(lists, 'the row came back with the read, not by reloading the list').toEqual([]);
 });
 
+test('artwork comes from iPX, kept, and only as an image', async ({ page }) => {
+  // Every image a feed names is drawn from iPX's address.
+  expect(await page.evaluate(() => artHTML('https://example.com/a.jpg', 'A'))).toContain('src="/api/art?u=https%3A%2F%2Fexample.com%2Fa.jpg"');
+  // Multi Show's items name art.jpg.
+  await expect(page.locator('.feed', { hasText: 'Multi Show' })).toBeVisible({ timeout: 20_000 });
+  const src = '/api/art?u=' + encodeURIComponent('http://127.0.0.1:8792/art.jpg');
+  for (const _ of [1, 2]) {                                         // fetched, then kept
+    const r = await page.request.get(src);
+    expect(r.status()).toBe(200);
+    expect(r.headers()['content-type']).toMatch(/^image\//);
+    expect(r.headers()['content-security-policy']).toContain('sandbox');
+  }
+  // An address no feed names is not fetched.
+  expect((await page.request.get('/api/art?u=' + encodeURIComponent('http://127.0.0.1:8792/show.xml'))).status()).toBe(404);
+});
+
 test('a deleted file looks as if it was never downloaded', async ({ page }) => {
   // Other people subscribe to Picture Blog by now, so both prompts come; take them.
   page.on('dialog', d => d.accept());