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.

backend/internal/store/store_test.go

@@ -543,6 +543,38 @@ func TestDisplayChapter(t *testing.T) {
 	}
 }
 
+func TestCoverURL(t *testing.T) {
+	cases := []struct {
+		name  string
+		cover string
+		want  string
+	}{
+		{
+			"kagane routes through the proxy",
+			"https://kagane.to/api/v2/image/019fe11a-84c3-7fc3-a84b-88787374b617/compressed",
+			"/img/kagane/019fe11a-84c3-7fc3-a84b-88787374b617",
+		},
+		{
+			"another site is served as stored",
+			"https://gg.asuracomic.net/storage/media/1/conversions/cover.webp",
+			"https://gg.asuracomic.net/storage/media/1/conversions/cover.webp",
+		},
+		{
+			"a lookalike host is not rewritten",
+			"https://evil.example/api/v2/image/019fe11a-84c3-7fc3-a84b-88787374b617/compressed",
+			"https://evil.example/api/v2/image/019fe11a-84c3-7fc3-a84b-88787374b617/compressed",
+		},
+		{"no cover stays empty", "", ""},
+	}
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			if got := (Bookmark{Cover: tc.cover}).CoverURL(); got != tc.want {
+				t.Errorf("CoverURL() = %q, want %q", got, tc.want)
+			}
+		})
+	}
+}
+
 func TestUpsertKindDefaultsToManga(t *testing.T) {
 	store := newTestStore(t)
 	got, err := store.Upsert(store.OwnerID(), Bookmark{

backend/internal/web/cover.go

@@ -0,0 +1,129 @@
+package web
+
+import (
+	"context"
+	"log"
+	"net/http"
+	"regexp"
+	"sync"
+	"time"
+)
+
+// CoverFetcher retrieves one kagane cover by image id. Satisfied by
+// latest.BrowserFetcher, and nil when BROWSER_WS_URL is unset — which leaves
+// kagane covers exactly as unavailable as they were before this endpoint
+// existed, rather than hanging a request on a fetcher that cannot run.
+type CoverFetcher interface {
+	Image(ctx context.Context, imageID string) (body []byte, contentType string, err error)
+}
+
+// coverIDRe matches the request path segment that becomes part of an outbound
+// URL. The proxy is session-gated, but the id still reaches a headless browser,
+// so it is validated at the boundary rather than passed through.
+var coverIDRe = regexp.MustCompile(`^[0-9a-f-]{36}$`)
+
+// coverTypes is the set of content types the proxy will echo back. A response
+// header sourced from a third party is not repeated verbatim: anything outside
+// this set is treated as "not a cover".
+var coverTypes = map[string]bool{
+	"image/webp": true,
+	"image/jpeg": true,
+	"image/png":  true,
+	"image/avif": true,
+	"image/gif":  true,
+}
+
+// coverTimeout bounds one proxied cover. Shorter than the fetcher's own
+// challenge budget on purpose: a browser page is waiting on this, and a cover
+// that has not arrived by now is better left as a broken slot than as a request
+// holding a connection open.
+const coverTimeout = 20 * time.Second
+
+// coverCacheMax caps the in-memory cover cache. Covers are immutable per image
+// id and a library holds tens of series, so this is a ceiling that is never
+// reached in practice; reaching it clears the map rather than evicting by age.
+//
+// ponytail: flush-on-full, not LRU. Swap it for an LRU if a library ever grows
+// past this and the flush starts costing refetches.
+const coverCacheMax = 500
+
+type cachedCover struct {
+	body        []byte
+	contentType string
+}
+
+type coverCache struct {
+	mu sync.Mutex
+	m  map[string]cachedCover
+}
+
+func (c *coverCache) get(id string) (cachedCover, bool) {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	v, ok := c.m[id]
+	return v, ok
+}
+
+func (c *coverCache) put(id string, v cachedCover) {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	if c.m == nil || len(c.m) >= coverCacheMax {
+		c.m = make(map[string]cachedCover, coverCacheMax)
+	}
+	c.m[id] = v
+}
+
+// kaganeCover serves a kagane cover from the backend's own origin.
+//
+// kagane answers image requests with a Cloudflare challenge and
+// `cross-origin-resource-policy: same-origin`, so the web UI cannot render one
+// directly under any combination of referrer policy or crossorigin attribute
+// (verified 2026-08-08). Fetching it through the headless browser that already
+// clears the challenge, and re-serving it here, is what puts the bytes on an
+// origin the page may load from.
+//
+// ponytail: covers are fetched on first view, one browser navigation at a time
+// behind the fetcher's mutex, so a first load of a large kagane library
+// trickles in over a few seconds. The cache makes it a one-off. Prefetching
+// during the poll cycle is the upgrade if that ever grates.
+func (h *Handler) kaganeCover(w http.ResponseWriter, r *http.Request) {
+	id := r.PathValue("id")
+	if !coverIDRe.MatchString(id) {
+		http.NotFound(w, r)
+		return
+	}
+	if h.covers == nil {
+		http.NotFound(w, r)
+		return
+	}
+	if v, ok := h.coverCache.get(id); ok {
+		writeCover(w, v)
+		return
+	}
+
+	ctx, cancel := context.WithTimeout(r.Context(), coverTimeout)
+	defer cancel()
+	body, contentType, err := h.covers.Image(ctx, id)
+	if err != nil {
+		log.Printf("kagane cover %s: %v", id, err)
+		http.NotFound(w, r)
+		return
+	}
+	if !coverTypes[contentType] {
+		log.Printf("kagane cover %s: unexpected content type %q", id, contentType)
+		http.NotFound(w, r)
+		return
+	}
+
+	v := cachedCover{body: body, contentType: contentType}
+	h.coverCache.put(id, v)
+	writeCover(w, v)
+}
+
+// writeCover sends the bytes with a long cache life: an image id names one
+// immutable rendering, so a client that has it never needs to ask again.
+func writeCover(w http.ResponseWriter, v cachedCover) {
+	w.Header().Set("Content-Type", v.contentType)
+	w.Header().Set("Cache-Control", "private, max-age=604800, immutable")
+	w.Write(v.body)
+}

backend/internal/web/templates/card.html

@@ -6,7 +6,7 @@
   <div class="row">
     <a class="cover" href="{{.ContinueURL}}" target="_blank" rel="noopener noreferrer"
        tabindex="-1" aria-hidden="true">
-      {{if .Cover}}<img src="{{.Cover}}" alt="" loading="lazy">
+      {{if .CoverURL}}<img src="{{.CoverURL}}" alt="" loading="lazy">
       {{/* aria-hidden on the cover link is not enough — Chromium still exposes
            the letter because the link is programmatically focusable — so the
            monogram carries its own, same as the recent strip's. */}}

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}}

backend/internal/web/web.go

@@ -50,6 +50,10 @@ type Handler struct {
 	// httpClient is the plain stdlib client that talks to Discord. It is not
 	// an injected interface: tests point APIBase at a stub server instead.
 	httpClient *http.Client
+	// covers proxies kagane cover images, which no browser can load directly.
+	// Nil disables the endpoint — see CoverFetcher.
+	covers     CoverFetcher
+	coverCache coverCache
 }
 
 // listView is what every list-rendering template receives.
@@ -111,7 +115,7 @@ type loginView struct {
 
 // New parses every template up front so a broken one kills the process at
 // startup rather than the first request that touches it.
-func New(s *store.Store, discord DiscordConfig, tokenKey []byte, mangaPath, novelPath string) (*Handler, error) {
+func New(s *store.Store, discord DiscordConfig, tokenKey []byte, mangaPath, novelPath string, covers CoverFetcher) (*Handler, error) {
 	tmpl, err := template.ParseFS(templateFS, "templates/*.html")
 	if err != nil {
 		return nil, err
@@ -126,6 +130,7 @@ func New(s *store.Store, discord DiscordConfig, tokenKey []byte, mangaPath, nove
 		states:              newOAuthStates(),
 		limiter:             session.NewLoginLimiter(),
 		httpClient:          &http.Client{Timeout: discordTimeout},
+		covers:              covers,
 	}, nil
 }
 
@@ -142,6 +147,10 @@ func (h *Handler) Register(mux *http.ServeMux) {
 	mux.HandleFunc("POST /ui/bookmarks/{key}/chapter", h.requireSession(h.uiChapter))
 	mux.HandleFunc("DELETE /ui/bookmarks/{key}", h.requireSession(h.uiDelete))
 
+	// Session-gated like every other UI route: the deployment proxies kagane's
+	// images for its own Readers, not for the internet.
+	mux.HandleFunc("GET /img/kagane/{id}", h.requireSession(h.kaganeCover))
+
 	// Install endpoints render the script directly under the session: the
 	// credential travels inside the served bytes, never in the address bar or
 	// the page markup. Updates after install use the credential-bearing /u/