fix newsletter api route

app/api/newsletter/route.ts

@@ -0,0 +1,9 @@
+import { NewsletterAPI } from 'pliny/newsletter'
+import siteMetadata from '@/data/siteMetadata'
+
+const handler = NewsletterAPI({
+  // @ts-ignore
+  provider: siteMetadata.newsletter.provider,
+})
+
+export { handler as GET, handler as POST }