Add core identity flow with persisted sessions and migrations

[rgomids]

Summary

• add the first functional identity slice with account creation, email/password login, and logout
• introduce users and identity module internals for users, accounts, credentials, and revocable sessions
• add Argon2 password hashing, JWT transport, SQL migrations, and global HTTP validation/error handling
• update README, architecture, commands, phase status, handoff, and changelog for Phase 1

Testing

• npm run lint
• npm run build
• npm run test:unit
• npm run test:integration (skipped locally because Docker was unavailable)
• npm run test:functional (skipped locally because Docker was unavailable)