fix(15-01): create backend_users before author_slug in models tests

TestPostTranslatableSmoke ran Plugin.Migrations() on an empty Postgres, so the author_slug ALTER failed. Match TestJournalTables and migrate lagoon backend_users first.

Co-authored-by: Cursor <cursoragent@cursor.com>

models/post_translatable_smoke_test.go

@@ -111,6 +111,13 @@ func journalDB(t *testing.T) *gorm.DB {
 
 func migrateTranslateThenJournal(t *testing.T, gdb *gorm.DB) {
 	t.Helper()
+	backend := gormigrate.New(gdb, &gormigrate.Options{
+		TableName:      "summer_migrations_summercms_cabana",
+		UseTransaction: true,
+	}, lagoon.BackendAdminMigrations)
+	if err := backend.Migrate(); err != nil {
+		t.Fatalf("backend_users migrations: %v", err)
+	}
 	translateMig := gormigrate.New(gdb, &gormigrate.Options{
 		TableName:      "summer_migrations_golem15_translate",
 		UseTransaction: true,