Keep a page error or a busy engine from blanking the app

A user on Upload got a white page while the engine's proxy answered
many requests with nginx's rate-limit 503.

- Pages render inside an error boundary, so a crash shows a message
  with Try again and Reload instead of unmounting the whole app.
- A 503 or 429 without the engine's JSON reads "The engine is busy",
  and queries wait 5s, then 10s, before asking again instead of 1s.
- Upload asks for knowledge details two at a time and reuses them
  until the document's knowledge status changes.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/api/engine.ts

@@ -1,5 +1,5 @@
 // Typed engine calls used by Express Mode. Ported from prototype/engine.js.
-import { EngineError, opKey, request, sleep } from './client'
+import { EngineError, limiter, opKey, request, sleep } from './client'
 import { t } from '../i18n'
 
 // ── Documents (Brain) ──
@@ -76,7 +76,10 @@ export interface KnowledgeDetail {
   error: string | null
 }
 
-export const knowledgeDetail = (id: number) => request<KnowledgeDetail>(`/upload/${id}/knowledge`)
+// The Upload page asks once per document waiting on an answer; two at a time keeps a long list
+// from tripping the proxy's rate limit.
+const twoAtATime = limiter(2)
+export const knowledgeDetail = (id: number) => twoAtATime(() => request<KnowledgeDetail>(`/upload/${id}/knowledge`))
 
 // Agree to the quoted price, build after a top-up, or retry a failed build. Spends workspace credits (402 when short).
 // Reads a failed document's stored file again, for free. A knowledge base it already has is kept.