fix(art): give both 2026-10-20 posts their artwork, and gate the build on it

The YuE2 and subtitles posts shipped text-only: render_from_art.mjs only READS
ogImage/banner, and after the 2026-10-19 bulk migration no script had to write
them, so both pages advertised /og-default.jpg and rendered no hero banner.

- new metaphors (in build_captions.py, frozen here via sync_prompts_catalog.py),
  captions, art-meta evidence, banner plan and the five derived files per slug
- frontmatter: ogImage + banner in EN and ZH for both posts
- scripts/check-assets.mjs is now step 1 of npm run build, so CI fails when a
  published post lacks a language-correct og card, banner, banner .webp or
  400/800 list thumb. It reported 16 problems on the unfixed tree, all here.

src/content/posts/zh/generating-subtitles-for-songs-without-timestamps.md

@@ -4,6 +4,8 @@ description: "AI 音乐工具只给你音频 —— 没有逐行时间，没有
 pubDate: 2026-10-20
 category: tutorials
 tags: ["subtitles", "lrc", "srt", "whisper", "forced-alignment", "ai-music", "python"]
+ogImage: "/og/zh-generating-subtitles-for-songs-without-timestamps.jpg"
+banner: "/banners/generating-subtitles-for-songs-without-timestamps.jpg"
 draft: false
 ---
 我试过的每一个 AI 音乐工具，交出来的东西形状都一样：一个音频文件。没有一个会告诉你**每一句是什么时候唱的**。只想听的话没问题，但只要你想把歌词放到屏幕上 —— 卡拉 OK 的那一行、视频字幕、播放器下面的滚动框 —— 就完全不够用。