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>