fix: skip down migrations in potentially_stale checksum comparison (#8271)

The potentially_stale block iterated over all migrations including
.down.sql reversible migrations. Down migrations share the same version
as their up counterpart but have a different checksum, causing the
DELETE to remove the up migration row on every startup and triggering
re-application of the concurrent index migrations.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

backend/windmill-api/src/db.rs

@@ -284,6 +284,9 @@ pub async fn migrate(
         20260207000004,
     ];
     for m in migrator.migrations.iter() {
+        if m.migration_type.is_down_migration() {
+            continue;
+        }
         if potentially_stale.contains(&m.version) {
             if let Err(err) =
                 sqlx::query("DELETE FROM _sqlx_migrations WHERE version = $1 AND checksum != $2")