fix(build): retry transient sitemap endpoint fetches

The TypeScript helper refactor exposed a separate production risk: builds can fail when the sitemap-endpoints API returns a transient 5xx during Pagefind index generation. Move that fetch into a small helper with retry logic and add a focused prebuild test so this failure mode is caught without waiting on a full deploy.

Constraint: Must keep Pagefind index generation behavior the same when the endpoint is healthy
Rejected: Ignore the failure as external-only | transient 5xx responses can still block deploys and CI repeatedly
Confidence: high
Scope-risk: narrow
Reversibility: clean
Directive: When external build inputs can fail transiently, add a small retryable helper plus a direct test before relying on full deploy verification
Tested: pnpm exec vitest run test/prebuild/load-sitemap-endpoints.test.ts; pnpm run typecheck; pnpm run test-prebuild
Not-tested: Full production redeploy completion before push

helpers/pagefind/load-sitemap-endpoints.ts

@@ -0,0 +1,63 @@
+import fs from 'fs-extra'
+import axios from 'axios'
+
+import {
+    sitemapEndpointsPath
+} from '~/helpers/pagefind/config.js'
+
+function shouldRetryError ( error: unknown ) {
+    const status = ( error as { response?: { status?: number } } )?.response?.status
+
+    return typeof status === 'number' && status >= 500
+}
+
+async function fetchJsonWithRetries (
+    url: string,
+    {
+        attempts = 3,
+        delayMs = 1000
+    }: {
+        attempts?: number
+        delayMs?: number
+    } = {}
+) {
+    let lastError: unknown
+
+    for ( let attempt = 1; attempt <= attempts; attempt += 1 ) {
+        try {
+            const response = await axios.get( url )
+
+            return response.data
+        } catch ( error ) {
+            lastError = error
+
+            if ( attempt >= attempts || !shouldRetryError( error ) ) {
+                throw error
+            }
+
+            await new Promise( resolve => setTimeout( resolve, delayMs ) )
+        }
+    }
+
+    throw lastError
+}
+
+export async function loadSitemapEndpoints () {
+    if ( await fs.pathExists( sitemapEndpointsPath ) ) {
+        return await fs.readJson( sitemapEndpointsPath )
+    }
+
+    if ( !process.env.PUBLIC_API_DOMAIN ) {
+        throw new Error(`Missing ${ sitemapEndpointsPath } and PUBLIC_API_DOMAIN is not set`)
+    }
+
+    const apiUrl = new URL( process.env.PUBLIC_API_DOMAIN )
+    apiUrl.pathname = sitemapEndpointsPath.replace(/^\.?\/?static\//, '/')
+
+    return await fetchJsonWithRetries( apiUrl.toString() )
+}
+
+export {
+    fetchJsonWithRetries,
+    shouldRetryError
+}