feat(art): replace all 72 posts' artwork with the v2 pipeline

Every post now carries one unique local-ComfyUI/Ideogram 4 base frame, composited
into EN + ZH share cards, a banner and list thumbs.

- the shipped prompts_catalog held only 21 distinct metaphors for 72 posts (16
  shared one visual), so all 72 were rewritten as unique object+state metaphors
  and 10 frames re-rolled after OCR found pseudo-text rendered at legible size
- render_from_art.mjs emits JPEG by default (558 KB PNG -> 84 KB mozjpeg q85);
  the ZH 1200x630 card did not exist before this batch
- 288 frontmatter fields migrated; the 144 v1 PNGs are gone
- PostList's thumb helper keys off the base slug (a ZH card is zh-<slug>.jpg while
  the thumb is <slug>-800.webp, which 404'd the whole ZH listing)
- [...slug].astro's <picture> regex accepts .jpg
- the repo now carries the art inputs (metaphors.json, captions/<slug>.json,
  refreshed prompts_catalog.json) so a frame is reproducible from git alone

src/content/posts/zh/how-i-made-my-own-songs-with-suno-ai.md

@@ -4,8 +4,8 @@ description: "自己写词、调风格提示词，用 Suno AI 做出七首原创
 pubDate: 2026-09-11
 category: case-studies
 tags: [suno, ai-music, music, lyrics, prompting, ai]
-ogImage: /og/how-i-made-my-own-songs-with-suno-ai.png
-banner: /banners/how-i-made-my-own-songs-with-suno-ai.png
+ogImage: /og/zh-how-i-made-my-own-songs-with-suno-ai.jpg
+banner: /banners/how-i-made-my-own-songs-with-suno-ai.jpg
 ---
 
 我做自己的歌。不是靠弹奏乐器——而是写歌词、打磨风格提示词，让 Suno AI