Update Phase 2 HTTP contracts, observability, and test coverage

[rgomids]

Summary

• standardize HTTP error responses to use error, message, and request_id
• add explicit request validation at the HTTP boundary for customers, invoices, and payments
• enrich request logging with module and request_id, and align bootstrap logging with the Phase 2 observability model
• expand unit, handler, integration, and functional coverage for validation, conflicts, immutability, and idempotency
• update Phase 2 documentation and governance artifacts across README.md, AGENTS.md, .agents/, commands, diagrams, and changelog

Testing

• make fmt
• make lint
• make test-unit
• make test-integration
• make test-functional
• go test ./...