add force-static

app/api/newsletter/route.ts

@@ -1,6 +1,8 @@
 import { NewsletterAPI } from 'pliny/newsletter'
 import siteMetadata from '@/data/siteMetadata'
 
+export const dynamic = 'force-static'
+
 const handler = NewsletterAPI({
   // @ts-ignore
   provider: siteMetadata.newsletter.provider,