Migrate existing Brevo contacts

[cka-y]

Description:
One-time migration task: pull all contacts currently on the Brevo announcements list, create corresponding notification_subscription rows (type api.announcements) in the database, and update each Brevo contact with MDB_SUBSCRIPTION_ID = <subscription_UUID>.

Should be idempotent and can reuse the pattern from migrate_firebase_users (PR #1716).

Acceptance Criteria:

• Script/task pulls all contacts from the Brevo announcements list
• Creates notification_subscription rows for each contact with type api.announcements
• Updates each Brevo contact's MDB_SUBSCRIPTION_ID attribute with the new subscription UUID
• Migration is idempotent (safe to re-run)
• Logging for audit trail