Fix tag_gpg_signatures project deletion

What does this MR do and why?

Problem

Project deletion fails with ActiveRecord::NotNullViolation because the tag_gpg_signatures.project_id foreign key uses ON DELETE SET NULL, but the column has a NOT NULL constraint. PostgreSQL cannot nullify the column on cascade.

Similar tables like ssh_signatures, tag_ssh_signatures and gpg_signatures already set ON DELETE CASCADE for project_id column. So tag_gpg_signatures is the only exception.

Solution

Replace the foreign key with ON DELETE CASCADE so that tag_gpg_signatures rows are deleted when the parent project is destroyed. The FK is added with validate: false and asynchronous validation is scheduled for a low-traffic window to avoid a long-running table scan during deployment.

A follow-up MR will add the synchronous validate_foreign_key migration after confirming the FK is valid in production.

References

• 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. Run bundle exec rspec spec/models/repositories/tags/gpg_signature_spec.rb.
3. Confirm db/structure.sql updates the tag_gpg_signatures.project_id foreign key from ON DELETE SET NULL to ON DELETE CASCADE NOT VALID.
4. Destroy a project with associated tag_gpg_signatures and verify deletion succeeds.

MR acceptance checklist

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