feat(erd): enum values on enum columns #1280

Part of #1286.

Problem

An enum column shows only the type name (order_status_enum); its values are what the reader needs.

Scope

• PostgreSQL: values from pg_enum for every enum type used by the shown tables; MySQL: values from column_type (enum('a','b')).
• The type is marked as an enum on the card; the tooltip lists the values (capped, with a count).
• Domains show their base type in the tooltip.

Acceptance

• PostgreSQL enum and MySQL enum columns list their values on hover.
• A type with many values is capped with "+N more".