docs: update multi-backend plan with implementation status

Updates based on design discussions:
- Mark Phase 1 (core type refactoring) as complete
- Reference feature/remove-unsigned-core-types branch
- Remove enum from core types (too different between backends)
- Document datetime→timestamp mapping for PostgreSQL
- Document json→jsonb mapping for PostgreSQL
- Add timezone policy (UTC storage, app-level conversion)
- Add Appendix C listing removed types with alternatives
- Streamline adapter interface (remove verbose implementation examples)

Author: claude