Remove oauth_consents client_id foreign key for cross-cell compatibility

Closes #597595 (closed)

What does this MR do and why?

What?

• Removes the oauth_consents foreign key fk_c5f142ff4b (oauth_consents.client_id → oauth_applications.uid) in a post-deploy migration, as agreed on the issue.

Why?

Before:

• DCR clients are created on the IAM service through POST /oauth/register and live only in IAM's database.
• If a user consents to an application's authorization:
  • GitLab creates a new oauth_consents row
  • If oauth_consents.client_id does not match any oauth_applications.uid, it throws a PG::ForeignKeyViolation
    • This happens because the oauth application is in the IAM table (oauth_clients)
• If a user deletes an oauth_application:
  • It automatically deletes its matching oauth_consents rows

After:

Found while testing the register endpoint: gitlab-org/auth/iam!560 (comment 3727417697)

• DCR clients are created on the IAM service through POST /oauth/register and live only in IAM's database.
• If a user consents to an application's authorization:
  • GitLab creates a new oauth_consents row
  • If oauth_consents.client_id does not match any oauth_applications.uid, it allows the authz consent.
• If a user deletes an oauth_application:
  • It does not delete its matching oauth_consents rows

Database Review

• db:check-migrations

Steps to validate

1. On master, in rails console, confirm a consent whose client_id has no local application row is rejected with ActiveRecord::InvalidForeignKey:

   Authn::OauthConsent.create!(
     user: User.first, client_id: 'iam-only-client-uid',
     consent_challenge: SecureRandom.hex, requested_scopes: ['mcp'],
     granted_scopes: ['mcp'], status: :authorized
   )

2. Check out this branch and run bin/rails db:migrate.

3. Run the same console snippet from step 1 again and confirm the consent persists this time.

MR acceptance checklist

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