IAV-798 : Refactor referral retrieval, add referral stats, paginate referred users, and...
Refactor referral retrieval, add referral stats, paginate referred users, and enhance Stripe first subscription handling
- Refactored getReferralByUserUid by removing the embedded list of ReferredDTO and introducing a dedicated referral statistics object
- Changed referred users retrieval to a paginated list via getReferredListPaginated
- Started creating a referral after the user's first successful subscription in StripeWebhookServiceImpl
- Updated first subscription logic to preserve first trial credits instead of removing them
Closes IAV-798