Render the PostgreSQL DOMAIN type in autogenerate

[cycsmail]

Fixes: #1360

Autogenerate had no renderer for the PostgreSQL DOMAIN type, so _repr_type() fell back to plain repr(). That dropped every constructor argument except name/data_type and left the underlying type unqualified (e.g. DOMAIN('email', CITEXT(), check="...") rendered as postgresql.DOMAIN('email', CITEXT())), silently losing the check constraint.

Adds _render_DOMAIN_type to PostgresqlImpl (which render_type already dispatches to via _render_<visit_name>_type). It renders the name, the underlying data_type through render._repr_type for the right prefix/import, and each of collation, default, constraint_name, not_null, check, create_type only when it differs from its default. The issue's column now renders as:

sa.Column('email', postgresql.DOMAIN('email', postgresql.CITEXT(),
    check="value ~ '^my_.*$'"), nullable=False)

Tests are guarded by config.requirements.sqlalchemy_2 since DOMAIN only exists on SQLAlchemy 2.0+. With the source change stashed both new tests fail (missing prefix and kwargs); with the fix they pass. pytest -q -k domain → 2 passed, and tests/test_postgresql.py tests/test_autogen_render.py → 249 passed, 2 skipped. Lint clean on the changed files.