feat(rust): staging apply + e2e for Postgres and MySQL #1992

Parent

#1849

Goal

Extend transactional staging apply beyond SQLite: apply_plan_postgres / apply_plan_mysql + live e2e (temp table → stage → generate → apply → assert → cleanup). Skip cleanly when live DB env is unavailable (QUERYA_SKIP_PG / QUERYA_SKIP_MYSQL).

Done when

• Success clears buffer; failure rolls back and leaves staging intact (parity with SQLite e2e)
• cargo test green offline (skips) and with live PG/MySQL

How to work

• Branch: issue/<n>-staging-apply-pg-mysql from epic/tauri-migration
• One PR → Closes #<n>, base epic/tauri-migration