fix: retry on 429 with backoff in BookStackClient.request() (#28)

## Summary

CI was hitting BookStack API rate limits when multiple pipelines ran
concurrently against the same instance (reproduced when opening several
PRs in quick succession).

`request()` now retries up to **3 times** on a 429 response, honouring
the `Retry-After` header (default 10 s) before each attempt. After 3
retries the 429 is thrown as normal.

## Behaviour

| Attempt | Response | Action |
|---|---|---|
| 1 | 429 | Wait `Retry-After` seconds, retry |
| 2 | 429 | Wait `Retry-After` seconds, retry |
| 3 | 429 | Wait `Retry-After` seconds, retry |
| 4 | 429 | Throw (status 429) |
| Any | 2xx | Return response |

## Test plan
- [ ] Functional tests pass without 429 errors on next CI run
- [ ] Unit tests covering retry behaviour added in follow-up PR
(`test/unit-tests`)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Paradoxbound <paradoxbound@paradoxbound.org>
Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: 3 people

packages/core/src/bookstack-client.ts

@@ -39,7 +39,7 @@ export class BookStackClient {
     path: string,
     params?: Record<string, string | number | undefined>,
     body?: unknown,
-    options?: { timeout?: number; returnText?: boolean }
+    options?: { timeout?: number; returnText?: boolean; _retries?: number }
   ): Promise<T> {
     let url = `${this.baseUrl}/api${path}`;
     if (params) {
@@ -66,6 +66,14 @@ export class BookStackClient {
       signal: controller.signal,
     });
     clearTimeout(timeoutId);
+    if (res.status === 429) {
+      const retriesLeft = options?._retries ?? 3;
+      if (retriesLeft > 0) {
+        const retryAfter = parseInt(res.headers.get('retry-after') ?? '10', 10);
+        await new Promise((resolve) => setTimeout(resolve, retryAfter * 1000));
+        return this.request(method, path, params, body, { ...options, _retries: retriesLeft - 1 });
+      }
+    }
     if (!res.ok) {
       const text = await res.text();
       let message = text;