feat(annotations): add polygon JSONB support for quadrilateral shapes

- 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>

backend/src/main/resources/db/migration/V23__add_polygon_to_annotations.sql

@@ -0,0 +1,8 @@
+-- Add optional polygon field for quadrilateral annotation shapes (Kraken OCR output).
+-- See ADR-002 for the design decision.
+
+ALTER TABLE document_annotations ADD COLUMN polygon JSONB;
+
+ALTER TABLE document_annotations
+ADD CONSTRAINT chk_annotation_polygon_quad
+    CHECK (polygon IS NULL OR jsonb_array_length(polygon) = 4);