feat(invoices): add Peppol delivery foundation (#1595)

* feat(invoices): add Peppol delivery foundation

* fix(invoices): harden Peppol compliance guards

* fix(api): narrow Peppol document loading

* test(pg): hash Peppol fixture payload

* fix(invoices): address Peppol review findings

* test(pg): isolate Peppol provider events

* test(pg): isolate Peppol submission fixtures

lib/api/private-no-store.ts

@@ -0,0 +1,6 @@
+import type { NextResponse } from 'next/server'
+
+export function privateNoStore(response: NextResponse): NextResponse {
+  response.headers.set('Cache-Control', 'private, no-store')
+  return response
+}