refactor: update PrimaryKeyMixin to use postgres uuidv7 defaults

[mplemay]

Summary

• switch PrimaryKeyMixin.id to rely on uuidv7() for Postgres server defaults and generate UUID4s via a SQLite-only before-flush hook
• strip uuidv7() from non-Postgres DDL and restore it afterward so SQLite migrations do not emit invalid defaults
• refresh the docstring and tests to describe the new behavior and validate the generated UUID4 after flush

Testing

• Not run (not requested)