Validate tag_gpg_signatures project_id foreign key

What does this MR do and why?

Contributes to !238703 (merged)

Problem

The tag_gpg_signatures.project_id foreign key was replaced with ON DELETE CASCADE in !238703 (merged) using validate: false and asynchronous validation. The FK remains marked as NOT VALID in PostgreSQL until synchronously validated.

Solution

Add a post-deployment migration that calls validate_foreign_key to synchronously validate the fk_ebf091e1c4 constraint, completing the two-step FK replacement process.

References

• Original MR: !238703 (merged)
• Sentry error: https://new-sentry.gitlab.net/organizations/gitlab/issues/3513874

Screenshots or screen recordings

Not applicable — no UI changes.

How to set up and validate locally

1. Run bundle exec rails db:migrate.
2. Confirm db/structure.sql updates the tag_gpg_signatures.project_id foreign key from ON DELETE CASCADE NOT VALID to ON DELETE CASCADE.

MR acceptance checklist

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