post: when a proxy timeout returns HTML instead of JSON (EN+ZH)

499 post-mortem: an O(n) library audit inside a polled GET on a one-core
cgroup, three reverse-proxy hops, and the status no monitor watches.

- measured: 13.6 s idle / 17.7-98.5-35.8 s probes / 76-2507 s under load
- evidence triad: 7x 499 @ portal, zero app-log entries, in-container repro
- fix: 300 s TTL cache + single-flight refresh + refreshing/cached_at flags,
  text-first client reader, ?refresh=1
- result: cached path 200 in 0.24-0.36 s (median 0.26 s)
- art: ideogram4 base frame + 4 derived targets, art-meta evidence line
- LinkedIn companion: docs/linkedin/batch-07-linkedin-posts.md

scripts/og-gen/art-meta.json

@@ -321,5 +321,15 @@
   "evidence": "89 covers in 48m · 35% safe zone · Laplacian filter",
   "ogTitle_en": "Batch Generating 80+ Technical Covers",
   "ogTitle_zh": "ComfyUI 批量闭环 80+ 张技术封面"
+ },
+ "when-a-proxy-timeout-returns-html-instead-of-json": {
+  "evidence": "13.6 s -> 0.26 s · 2507 s worst · 499 at the gate",
+  "ogTitle_en": "The Proxy That Replied in HTML",
+  "ogTitle_zh": "反代用 HTML 回答了 JSON",
+  "tags": [
+   "nginx",
+   "timeouts",
+   "caching"
+  ]
  }
 }