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/store/store.go

@@ -137,6 +137,22 @@ func (b Bookmark) Initial() string {
 	return "?"
 }
 
+// kaganeCoverRe matches the cover URL kagane's og:image carries, which is what
+// the userscript stores for that site.
+var kaganeCoverRe = regexp.MustCompile(`^https://kagane\.to/api/v2/image/([0-9a-f-]{36})/compressed$`)
+
+// CoverURL is the src the web UI puts in an <img>. For every site but kagane
+// that is Cover as stored. kagane serves its images behind a Cloudflare
+// challenge *and* with `cross-origin-resource-policy: same-origin`, so no page
+// on another origin can load one however it asks (verified 2026-08-08); those
+// go through the backend's own proxy instead.
+func (b Bookmark) CoverURL() string {
+	if m := kaganeCoverRe.FindStringSubmatch(b.Cover); m != nil {
+		return "/img/kagane/" + m[1]
+	}
+	return b.Cover
+}
+
 // Library buckets. A bookmark is in exactly one. This cannot be derived from
 // Site: asurascans serves manga and novels from the same /comics/ path, so the
 // userscript that recorded the page is the only party that knows which.