Time out a hung feed, serve the precomposed touch icon, and store http artwork on https (#108, #109, #110)

#108: the HTTP client had no timeout, and scans handle feeds in order, so a hung server held
every scan. Dreamwidth answered 504 after 60-67 s for a day and each scan took 70-80 s instead of
15. A feed fetch, and a Patreon creator's show list, now gets 30 s from connecting to the last
byte (feed::FEED_TIMEOUT); the client gets a 10 s connect timeout, which bounds a download's
start but not a long download.

#109: iOS asks for /apple-touch-icon-precomposed.png first when the site is added to a home
screen; it was a 404 and the only non-feed warning in the log. It serves the same icon.

#110: the page is https and loads no http. Artwork on http came through /api/art (#90) even
when its host serves https too. A scan now tries each http artwork host on https once per feed
(feed::prefer_https) and stores the https address where the host answers with an image,
rewriting that feed's stored items from the same host (Db::secure_images). 4 of the 5 hosts in
production do; cdn.thesecretcabal.com presents another name's certificate and stays on
/api/art.

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

tests/ui/app.spec.js

@@ -1233,7 +1233,7 @@ test('the favicon is the logo, square, from both pages', async ({ page }) => {
   await page.evaluate(() => setTheme('modern', 'dark'));
   await expect(page.locator('#favicon')).toHaveAttribute('href', /^\/favicon-dark\.png\?v=[0-9a-f]{12}$/);
   // A browser asks for /favicon.ico on its own, signed in or not.
-  for (const path of ['/favicon.ico', '/favicon.png', '/favicon-dark.png', '/apple-touch-icon.png']) {
+  for (const path of ['/favicon.ico', '/favicon.png', '/favicon-dark.png', '/apple-touch-icon.png', '/apple-touch-icon-precomposed.png']) {
     const r = await page.request.get(path, { headers: { cookie: '' } });
     expect(r.status(), path).toBe(200);
     expect(r.headers()['content-type'], path).toBe('image/png');