refactor(billing): consolidate billing duplication (Commit 2.4)

- Create billing-core.ts with shared billing logic
- Extract calculateUsageAndBalanceFromGrants, getOrderedActiveGrantsForOwner
- Add GRANT_ORDER_BY constant for consistent grant ordering
- Update balance-calculator.ts and org-billing.ts to delegate to core
- Add comprehensive unit tests for billing calculations

Author: brandonkachen