fix: resolve TypeScript strict null errors in producer

- server.ts: default token param to empty string to satisfy string type
- renderOrchestrator.ts: capture fileServer in local const before closure
  to preserve TypeScript null narrowing

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

packages/producer/src/server.ts

@@ -391,7 +391,7 @@ export function createRenderHandlers(options: HandlerOptions = {}): RenderHandle
   };
 
   const outputs = (c: Context): Response => {
-    const token = c.req.param("token");
+    const token = c.req.param("token") ?? "";
     const artifact = store.get(token);
     if (!artifact) {
       return c.json({ success: false, error: "Output artifact not found or expired" }, 404);