feat(web): proxy kagane cover images so the UI can render them

kagane serves cover images from behind the same Cloudflare challenge as
its pages and with cross-origin-resource-policy: same-origin. The second
header is the decisive one: no <img> on the web UI's origin can load a
kagane cover even from a browser that already holds the clearance cookie,
verified 2026-08-08 by loading one from a foreign origin with and without
a referrer. Hot-linking cannot be made to work, so every kagane series
rendered the monogram placeholder.

Bookmark.CoverURL rewrites a stored kagane og:image to /img/kagane/{id}
and returns every other cover untouched; the templates render .CoverURL
in place of .Cover. The endpoint is session-gated like every other UI
route, and hands the id to the shared headless browser, whose fetch is
same-origin with kagane and therefore satisfies both the challenge and
the CORP header. Results are memoised in-process, so a cover costs one
navigation per deployment lifetime.

The id is matched against a UUID regex before it reaches the browser.
That gate is load-bearing rather than tidiness: the cover is a stored
client-supplied string, so an unvalidated one turns the endpoint into an
SSRF primitive aimed at the deployment's own network. ServeMux
path-cleans a traversal into a redirect before the handler runs, but the
handler does not rely on that, and a test pins it.

With BROWSER_WS_URL unset there is no browser and the endpoint answers
404 rather than reaching for a nil fetcher - the same degrade-to-
userscript behaviour the poller already has for these sites.

backend/internal/web/templates/chrome.html

@@ -14,7 +14,7 @@
     <a class="recent-card {{if .HasNewChapter}}is-new{{end}}" href="{{.ContinueURL}}"
        target="_blank" rel="noopener noreferrer">
       <span class="recent-cover">
-        {{if .Cover}}<img src="{{.Cover}}" alt="" loading="lazy">
+        {{if .CoverURL}}<img src="{{.CoverURL}}" alt="" loading="lazy">
         {{else}}<span class="monogram" aria-hidden="true">{{.Initial}}</span>{{end}}
         {{if .HasNewChapter}}<span class="foot-rule"></span>
         {{else if .Favorite}}<span class="foot-rule brass"></span>{{end}}