fix(auth): add CORS headers to BetterAuth raw HTTP handler

The auth handler uses Fastify's onRequest hook with reply.hijack(),
which bypasses all NestJS middleware including CORS. Manually set
Access-Control-Allow-* headers on the raw response and handle OPTIONS
preflight before delegating to BetterAuth's node handler.

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

apps/gateway/src/auth/auth.controller.ts

@@ -7,16 +7,17 @@ import { AUTH } from './auth.tokens.js';
 export function mountAuthHandler(app: NestFastifyApplication): void {
   const auth = app.get<Auth>(AUTH);
   const nodeHandler = toNodeHandler(auth);
+  const corsOrigin = process.env['GATEWAY_CORS_ORIGIN'] ?? 'http://localhost:3000';
 
   const fastify = app.getHttpAdapter().getInstance();
 
-  // Use Fastify's addHook to intercept auth requests at the raw HTTP level,
-  // before Fastify's body parser runs. This avoids conflicts with NestJS's
-  // custom content-type parser.
+  // BetterAuth is mounted at the raw HTTP level via Fastify's onRequest hook,
+  // bypassing NestJS middleware (including CORS). We must set CORS headers
+  // manually on the raw response before handing off to BetterAuth.
   fastify.addHook(
     'onRequest',
     (
-      req: { raw: IncomingMessage; url: string },
+      req: { raw: IncomingMessage; url: string; method: string },
       reply: { raw: ServerResponse; hijack: () => void },
       done: () => void,
     ) => {
@@ -25,6 +26,27 @@ export function mountAuthHandler(app: NestFastifyApplication): void {
         return;
       }
 
+      const origin = req.raw.headers.origin;
+      const allowed = corsOrigin.split(',').map((o) => o.trim());
+
+      if (origin && allowed.includes(origin)) {
+        reply.raw.setHeader('Access-Control-Allow-Origin', origin);
+        reply.raw.setHeader('Access-Control-Allow-Credentials', 'true');
+        reply.raw.setHeader(
+          'Access-Control-Allow-Methods',
+          'GET, POST, PUT, PATCH, DELETE, OPTIONS',
+        );
+        reply.raw.setHeader('Access-Control-Allow-Headers', 'Content-Type, Authorization, Cookie');
+      }
+
+      // Handle preflight
+      if (req.method === 'OPTIONS') {
+        reply.hijack();
+        reply.raw.writeHead(204);
+        reply.raw.end();
+        return;
+      }
+
       reply.hijack();
       nodeHandler(req.raw as IncomingMessage, reply.raw as ServerResponse)
         .then(() => {