Browser-backed Sites join the Cover pipeline (#62)

backend/internal/latest/cover.go

@@ -22,6 +22,29 @@ type CoverBytesFetcher interface {
 	Fetch(ctx context.Context, sourceURL string) (body []byte, contentType string, err error)
 }
 
+// fetchCoverBytes routes a cover's byte retrieval by Site: only kagane needs
+// the browser for image bytes — its covers answer a plain fetch with a
+// challenge and `cross-origin-resource-policy: same-origin` — while every
+// other Site's CDN answers plain TLS. Missing fetchers degrade to an error the
+// caller logs, never a fallback onto a path that cannot succeed. One routing
+// rule for the poll and the acquirer, so the two cannot drift apart.
+func fetchCoverBytes(ctx context.Context, site, cover string, browser BrowserCoverFetcher, tls CoverBytesFetcher) ([]byte, string, error) {
+	if site == "kagane" {
+		if browser == nil {
+			return nil, "", errors.New("no cover fetcher")
+		}
+		imageID, ok := store.KaganeImageID(cover)
+		if !ok {
+			return nil, "", errors.New("invalid kagane cover URL")
+		}
+		return browser.Image(ctx, imageID)
+	}
+	if tls == nil {
+		return nil, "", errors.New("no cover fetcher")
+	}
+	return tls.Fetch(ctx, cover)
+}
+
 // CoverResolver resolves a host before any connection is attempted. Tests
 // inject it to exercise hostile DNS results without touching the live network.
 type CoverResolver func(context.Context, string) ([]netip.Addr, error)