docs: add multi-backend support design plan (MySQL + PostgreSQL)

Comprehensive implementation plan for adding PostgreSQL support:

- Adapter pattern architecture with DatabaseAdapter ABC
- Core types (portable) vs native types (backend-specific)
- Remove unsigned integers from core types (not portable)
- Type mapping tables for MySQL and PostgreSQL
- Phased implementation checklist
- Testing strategy with Docker Compose and CI matrix
- Migration tools for schema portability

Key design decisions:
- Core types work identically across all backends
- Native types pass through verbatim (backend-specific)
- Users explicitly choose portability vs backend features

Author: claude