Remove free credits system (signup bonus, daily credits) and B2B organization
credits to simplify the codebase. Credits now only come from purchases or gifts.
Changes:
- Remove freeCreditsRemaining, dailyFreeCredits, lastDailyResetAt from balances
- Remove organizationBalances and creditAllocations tables from schema
- Simplify transaction types to: purchase, usage, refund, gift
- Remove B2B endpoints from credits controller
- Remove checkDailyReset, allocateCredits, deductCredits from service
- Add redeemPendingGifts method to auto-redeem gifts on registration
- Update frontend to remove free credits display
- Add database migration for the changes
- Update all related tests to match simplified system
- Add gift code creation, redemption, and refund endpoints
- Add Stripe payment link generation for credits
- Add gifts database schema
- Enhance credits controller with new operations