docs: add Aurora PostgreSQL Express support plan

Aurora PostgreSQL Express Configuration (VPC-less, GA 2026-03-25) resolves
major DSQL pain points (no FK, no JSON, no SERIAL, 1 DDL/tx, ALTER
constraints). This document captures the analysis and plan for supporting
both engines in the starter kit.

Key decisions:
- Two-branch model (main=Aurora Express, dsql=DSQL) instead of if-branching
- engine/ directory convention to isolate DB-specific code from shared code
- Wait for CloudFormation Express Config support before releasing Aurora
  Express on main; publish DSQL as v3.0.0-dsql branch in the meantime
- Use drizzle-orm built-in migrate() instead of custom migration runner

Includes: DSQL vs Aurora Express trade-off analysis, CloudFormation schema
investigation (describe-type confirmed no Express Config properties),
cost comparison, conflict analysis for two-branch maintenance, and
phased rollout plan.

Author: konokenj