Backport of 'Fix keys.organization_id NOT NULL constraint failure on Self-Managed/Dedicated upgrades' to 19.1

What does this MR do and why?

Backports !245539 (merged) (fix for #605197 (closed)) to 19-1-stable-ee.

The post-deploy migration 20260422180000 (milestone 19.0) crashes upgrades with:

PG::CheckViolation: check constraint "check_8933ae4f38" of relation "keys" is violated by some row

Because it raises, no subsequent migrations run, halting the upgrade. This is blocking GitLab Dedicated tenants upgrading into 19.x (SRE incident on #605197 (closed)). 19.0.x is the urgent target; 19.1 and 19.2 are backported alongside so tenants on those versions are also covered. Manual per-tenant SQL patching does not scale to the fleet.

The fix no-ops the failing 20260422180000 and re-introduces the backfill (20260714092405) and NOT NULL constraint (20260714092408) as new post-deploy migrations. The migration milestone is set to 19.1 for this stable branch.

⚠️ Do not merge until !245539 (merged) has been deployed to GitLab.com.

• Master MR: !245539 (merged)
• Issue: #605197 (closed)
• Sibling precedent: !242492 (merged) (#600899 (closed), oauth_applications)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
• The MR that fixed the bug on the default branch has been deployed to GitLab.com (not applicable for documentation or spec changes).
• The MR title is descriptive (e.g. "Backport of 'title of default branch MR'"). This is important, since the title will be copied to the patch blog post.
• Required labels have been applied to this merge request
  • severity label and bug subtype labels (if applicable)
  • If this MR fixes a bug that affects customers, the customer label has been applied.
• This MR has been approved by a maintainer (only one approval is required).
• Ensure the e2e:test-on-omnibus-ee job has succeeded, or if it has failed, investigate the failures. If you determine the failures are unrelated, you may proceed. If you need assistance investigating, request help in the #s_developer_experience Slack channel to confirm the failures are unrelated to the merge request.

Note to the merge request author and maintainer

If you have questions about the patch release process, please:

• Refer to the patch release runbook for engineers and maintainers for guidance.
• Ask questions on the #releases Slack channel (internal only).
• Once the backport has been merged, the commit changes will be automatically deployed to a release environment that can be used for manual validation. See after merging runbook for details.