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>