Add database validations for started_at and uniqueness for namespace_id & subscription_add_on_id

Growth & Fulfillment need to sync on the following changes that were highlighted in the discussion in !164572 (comment 2116064746):

1. Database validation for started_at not null
2. Uniqueness on namespace_id & subscription_add_on_id (For this one we do have an index, just not a unique index)

Implementation Notes/Requirements

• Use created_at to backfill started_at. (Note: created_at is not used. This we can use that value to see which data has been migrated. All paid add on purchases will have accurate started_at in the future)