Validate imported rows and roll back on partial failure #1180

Background

Partial import failures leave the target in an inconsistent state. Validate all rows up front and roll back atomically.

Files to modify (2-3)

src/lib/import/validator.ts
src/lib/import/rollback.ts

Acceptance Criteria

Implemented across the listed files

Unit/integration tests added or updated and passing

No regression; follows project coding standards