fix(build): payment-file route returns a Uint8Array body, not a Buffer (#1850)

The zero-extensions build (tsconfig.build.json) rejects a Node Buffer as
a Response body (TS2345: Buffer<ArrayBufferLike> is not assignable to
BodyInit), which has made every PR's Core Build red since #1845 merged.
Main's push CI does not run that job, so it only surfaced on PRs. Same
pattern as the PDF/XLSX report routes.


Claude-Session: https://claude.ai/code/session_01ScVhg6XsDtNXkiEQNV7LaZ

Co-authored-by: Jakob Wennberg <311770904+jakobwennberg-oss@users.noreply.github.com>
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

app/api/skatteverket/tax-payments/[period]/payment-file/route.ts

@@ -218,7 +218,9 @@ export const GET = withRouteContext<{ params: Promise<{ period: string }> }>(
     .eq('id', agi.id)
     .eq('company_id', companyId)
 
-  return new Response(fileContent, {
+  // Buffer is not assignable to BodyInit under the strict build tsconfig
+  // (Buffer<ArrayBufferLike>); the repo convention is a Uint8Array view.
+  return new Response(new Uint8Array(fileContent), {
     headers: {
       'Content-Type': contentType,
       'Content-Disposition': `attachment; filename="${filename}"`,