Scaffolding: introspect CHECK constraints and enum types

[WailedParsley36]

PostgresSchemaReader reconstructs tables, columns, primary keys, unique constraints, and foreign keys, but CHECK constraints and database enum types are not yet read. Add:

• CHECK constraints via information_schema.check_constraints (skipping the synthesized NOT NULL checks).
• Enum types via pg_type/pg_enum, mapped to [Table]-backed enums / InstantiatedValues.

Cover with the existing live-DB round-trip test (PostgresSchemaReaderTests).