Add core identity foundation with persisted sessions and operational docs

[rgomids]

Summary

• add the Phase 1 core identity slice with accounts, credentials, sessions, and JWT-based authentication
• introduce the initial users module contract consumed by identity with explicit PostgreSQL repositories and migrations
• wire global validation, error handling, database lifecycle, and structured logging for auth flows
• update README, architecture, commands, handoff, and phase status docs to reflect the new foundation

Testing

• npm run build
• npm run test:unit
• npm run test:integration
• npm run test:functional