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/generating-subtitles-for-songs-without-timestamps.md

@@ -4,6 +4,8 @@ description: "AI music tools give you audio and nothing else — no line timings
 pubDate: 2026-10-20
 category: tutorials
 tags: ["subtitles", "lrc", "srt", "whisper", "forced-alignment", "ai-music", "python"]
+ogImage: "/og/generating-subtitles-for-songs-without-timestamps.jpg"
+banner: "/banners/generating-subtitles-for-songs-without-timestamps.jpg"
 draft: false
 ---
 Every AI music tool I have tried returns the same shape of output: an audio

src/content/posts/yue2-on-a-12gb-gpu-songs-that-lost-their-endings.md

@@ -4,6 +4,8 @@ description: "Twelve Mandarin songs came out exactly 240.000 seconds long with t
 pubDate: 2026-10-20
 category: ai
 tags: ["yue2", "comfyui", "ai-music", "local-ai", "gpu", "vram", "mandarin"]
+ogImage: "/og/yue2-on-a-12gb-gpu-songs-that-lost-their-endings.jpg"
+banner: "/banners/yue2-on-a-12gb-gpu-songs-that-lost-their-endings.jpg"
 draft: false
 ---
 YuE2 is a 3B open-weights music model that sings Mandarin from a style prompt

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。

src/content/posts/zh/yue2-on-a-12gb-gpu-songs-that-lost-their-endings.md

@@ -4,6 +4,8 @@ description: "12 首中文歌时长全都恰好 240.000 秒，尾奏不见了。
 pubDate: 2026-10-20
 category: ai
 tags: ["yue2", "comfyui", "ai-music", "local-ai", "gpu", "vram", "mandarin"]
+ogImage: "/og/zh-yue2-on-a-12gb-gpu-songs-that-lost-their-endings.jpg"
+banner: "/banners/yue2-on-a-12gb-gpu-songs-that-lost-their-endings.jpg"
 draft: false
 ---
 YuE2 是一个 30 亿参数的开源音乐模型，能按风格提示词和歌词唱中文。我把它放在自己那台 12GB 显存的 RTX 3060 上用 ComfyUI 本地跑，一次排了 12 首原创歌 —— 结果第一批出来，**每一首的时长都恰好是 240.000 秒**。这个数字不是巧合，而且是个坏消息。