feat: add DI support to organization billing functions with tests

- Add DI support to consumeOrganizationCredits via deps.withSerializableTransaction
- Add DI support to grantOrganizationCredits via deps.db
- Remove TODO comments that were added in previous commit
- Add 11 integration tests for consumeOrganizationCredits:
  - Priority order consumption
  - Multi-grant consumption
  - No grants error handling
  - Purchased credits tracking
- Add 6 integration tests for grantOrganizationCredits:
  - Correct values creation
  - Default description
  - Idempotency (duplicate handling)
  - Error propagation
  - Priority setting
  - Expiration date

104 billing tests now pass with full DI coverage.

Author: brandonkachen