Fix hosted_plan_name_uid for gitlab_subscriptions

What does this MR do and why?

This MR introduces a batched background migration to fix hosted_plan_name_uid values in the gitlab_subscriptions table that became out of sync due to a deployment gap.

The previous backfill migration (BackfillGitlabSubscriptionsHostedPlanNameUid, queued in 20260115114900) was finalized in !231589 (merged), but a deployment issue left some rows with hosted_plan_name_uid values that do not match the corresponding plans.plan_name_uid. This migration corrects those 2193 rows by re-syncing hosted_plan_name_uid from the plans table for any subscription where the values are mismatched or NULL.

This is part of the broader effort to make hosted_plan_name_uid the source of truth for plan associations in gitlab_subscriptions, removing the dependency on the plans table's id column.

Query plan

I used a subquery to get 1000 subscription ids but in practice those would be substituted in as a list before the query hits the DB.

https://console.postgres.ai/gitlab/gitlab-production-main/sessions/51488/commands/152148

References

• Related to #596996
• Parent issue: #571422
• Previous backfill: !231589 (merged)
• Source of truth migration: !230811 (merged)

Screenshots or screen recordings

N/A — background migration with no UI changes.

Before	After

How to set up and validate locally

1. Create gitlab_subscriptions rows with mismatched or NULL hosted_plan_name_uid values.
2. Run the migration: bundle exec rails db:migrate:post_migrate.
3. Verify that hosted_plan_name_uid now matches plans.plan_name_uid for all affected rows.

MR acceptance checklist

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