fix: give covers their own 10 MiB byte cap (#71) (#112)

Co-authored-by: Sulthan Zaki <sultankiki05@gmail.com>
Co-committed-by: Sulthan Zaki <sultankiki05@gmail.com>

backend/internal/latest/cover.go

@@ -47,6 +47,15 @@ func fetchCoverBytes(ctx context.Context, cover string, browser BrowserCoverFetc
 // inject it to exercise hostile DNS results without touching the live network.
 type CoverResolver func(context.Context, string) ([]netip.Addr, error)
 
+// maxCoverBytes caps one cover, separately from the series-page maxBodyBytes:
+// a cover is a bounded binary asset, not a text page, and 4 MiB rejected 12%
+// of asurascans covers measured 2026-08-17 (p90 4.52 MB, max 8.57 MB — two of
+// the three over-cap files were JPEGs, not the animated GIF of issue #71).
+// 10 MiB is ~18% headroom over that worst case and matches the GitHub and
+// Discord image limits; see docs/research/gif-maximum-byte-size.md. GIF itself
+// has no maximum size, so this number is policy, not format.
+const maxCoverBytes = 10 << 20
+
 // TLSCoverFetcher retrieves image bytes with the standard HTTPS client. Unlike
 // TLSFetcher, it does not need a browser fingerprint: cover hosts are public
 // CDNs and the response is accepted only after the destination gate passes.
@@ -152,15 +161,15 @@ func (f *TLSCoverFetcher) Fetch(ctx context.Context, sourceURL string) ([]byte,
 	if !ok {
 		return nil, "", fmt.Errorf("fetch cover: unsupported content type %q", raw)
 	}
-	if resp.ContentLength > maxBodyBytes {
-		return nil, "", fmt.Errorf("fetch cover: response exceeds %d bytes", maxBodyBytes)
+	if resp.ContentLength > maxCoverBytes {
+		return nil, "", fmt.Errorf("fetch cover: response exceeds %d bytes", maxCoverBytes)
 	}
-	body, err := io.ReadAll(io.LimitReader(resp.Body, maxBodyBytes+1))
+	body, err := io.ReadAll(io.LimitReader(resp.Body, maxCoverBytes+1))
 	if err != nil {
 		return nil, "", fmt.Errorf("read cover: %w", err)
 	}
-	if len(body) > maxBodyBytes {
-		return nil, "", fmt.Errorf("fetch cover: response exceeds %d bytes", maxBodyBytes)
+	if len(body) > maxCoverBytes {
+		return nil, "", fmt.Errorf("fetch cover: response exceeds %d bytes", maxCoverBytes)
 	}
 	return body, contentType, nil
 }