2a107a42f3
- New DB models: credit_packages, subscription_plans, user_credits, credit_consumptions, credit_purchases - CreditService: balance, deduct, add_credits, grant_free_trial, history - User API: /api/v1/credits/* (balance/history/packages/purchase/subscribe) - Admin API: /api/v1/admin/credit-* (CRUD packages/plans, user credits, consumptions) - PaymentService.create_credit_order + handle_callback for credit purchases - Credit deduction on: discovery, translate, marketing, ai_chat, followup - Free trial 30 credits on registration - Documentation: docs/CREDIT_SYSTEM.md