Description

Build a credit/account balance system where users can prepay funds and apply credits to subscriptions, with expiration and transfer capabilities.

Acceptance Criteria

• Account balance CRUD with ledger entries
• Credit purchase with multiple payment methods
• Auto-apply credit to upcoming invoices
• Credit expiration with configurable policy
• Credit transfer between users
• Balance history with running total

Technical Scope

• contracts/subscription/src/lib.rs
• src/store/subscriptionStore.ts
• src/screens/SubscriptionDetailScreen.tsx
• Edge: credit race condition (double spend), expiration notification, partial credit application