- V23 migration adds polygon JSONB column with 4-point CHECK constraint - PolygonConverter: AttributeConverter for List<List<Double>> <-> JSONB - @UniquePoints custom validator rejects duplicate coordinates - CreateAnnotationDTO: validated optional polygon field - DocumentAnnotation entity: polygon field with converter Refs #227 Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>