fix(backend): use redirect:'manual' in frontend API proxy fetch (#8186)

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: brkalow

.changeset/fix-proxy-redirect-manual.md

@@ -0,0 +1,5 @@
+---
+'@clerk/backend': patch
+---
+
+Fix frontend API proxy following redirects server-side instead of passing them to the browser. The proxy's `fetch()` call now uses `redirect: 'manual'` so that 3xx responses from FAPI (e.g. after OAuth callbacks) are returned to the client as-is, matching standard HTTP proxy behavior.

packages/backend/src/proxy.ts

@@ -278,6 +278,7 @@ export async function clerkFrontendApiProxy(request: Request, options?: Frontend
     const fetchOptions: RequestInit = {
       method: request.method,
       headers,
+      redirect: 'manual',
       // @ts-expect-error - duplex is required for streaming bodies but not in all TS definitions
       duplex: hasBody ? 'half' : undefined,
     };