feat(typeorm): standardize transaction isolation levels

[kauandotnet]

Summary

• replace TypeOrmCrudTransactionIsolationLevel string enum with a same-name const object and derived union type
• use TypeORM-native uppercase isolation values while preserving ReadCommitted / RepeatableRead member syntax
• pass isolation values directly to TypeORM transaction APIs and document the compatibility
• cover source types, runtime behavior, PostgreSQL integration, and isolated packed consumers

Compatibility

This intentionally changes the runtime string values from lowercase to TypeORM-native uppercase. Member-based consumers keep the same source syntax, while consumers persisting or comparing the old lowercase values must migrate. The supported CRUD subset remains READ COMMITTED and REPEATABLE READ.

Verification

• pnpm exec tsc --noEmit
• pnpm run check
• pnpm run test (323 tests)
• PG_URL=postgresql://nestm:nestm@localhost:55435/nestm_crud pnpm run test:postgres (65 tests)
• pnpm run verify:pack
• pnpm run verify:consumer