Add explicit type for policy_type

What does this MR do and why?

Add explicit type for policy_type

• Explicitly defining the attribute type for the enum column to prevent schema loading errors Sidekiq contexts by providing type information before the database schema is fully initialized.

References

• https://new-sentry.gitlab.net/organizations/gitlab/issues/1985815/?alert_rule_id=12&alert_type=issue&notification_uuid=0ebad097-9cc7-4779-8f6d-00f41e6dd4e0&project=3&referrer=slack

Screenshots or screen recordings

n/a

How to set up and validate locally

0. Still, figuring that out myself

MR acceptance checklist

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