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/client.test.ts

@@ -0,0 +1,19 @@
+import { describe, expect, it } from 'vitest'
+import { limiter } from './client'
+
+describe('limiter', () => {
+  it('runs at most n calls at once, all of them, failures included', async () => {
+    const limit = limiter(2)
+    let running = 0, peak = 0
+    const call = (n: number) => limit(async () => {
+      peak = Math.max(peak, ++running)
+      await new Promise(r => setTimeout(r, 5))
+      running--
+      if (n === 2) throw new Error('503')
+      return n
+    })
+    const out = await Promise.allSettled([1, 2, 3, 4, 5].map(call))
+    expect(peak).toBe(2)
+    expect(out.map(r => (r.status === 'fulfilled' ? r.value : 'x'))).toEqual([1, 'x', 3, 4, 5])
+  })
+})