fix(annotations): use @JdbcTypeCode(JSON) for polygon JSONB column

Replace @Convert(PolygonConverter) with Hibernate native @JdbcTypeCode(SqlTypes.JSON)
to fix JDBC type mismatch — PostgreSQL requires jsonb type, not varchar.

The PolygonConverter is retained as a standalone utility but no longer
used on the entity. Hibernate 6 natively handles List<List<Double>>
serialization to JSONB.

Refs #227

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/src/main/java/org/raddatz/familienarchiv/model/DocumentAnnotation.java

@@ -4,6 +4,8 @@ import io.swagger.v3.oas.annotations.media.Schema;
 import jakarta.persistence.*;
 import lombok.*;
 import org.hibernate.annotations.CreationTimestamp;
+import org.hibernate.annotations.JdbcTypeCode;
+import org.hibernate.type.SqlTypes;
 
 import java.time.LocalDateTime;
 import java.util.List;
@@ -53,8 +55,8 @@ public class DocumentAnnotation {
     @Column(name = "file_hash", length = 64)
     private String fileHash;
 
+    @JdbcTypeCode(SqlTypes.JSON)
     @Column(columnDefinition = "jsonb")
-    @Convert(converter = PolygonConverter.class)
     private List<List<Double>> polygon;
 
     @Column(name = "created_by")