Test billing engine lifecycle and totals

[solomon2773]

Summary

• add Vitest, coverage, and PostgreSQL Testcontainers infrastructure
• extract deterministic billing policy for totals, coupon boundaries, currency conversion, and proration
• cover renewal horizons and service lifecycle cutoffs against a real PostgreSQL schema
• make invoice payment claiming race-safe and run unit/integration suites in CI

Validation

• 12 unit tests
• 4 PostgreSQL integration tests
• strict TypeScript check
• production Next.js build

Closes #5