#7322 fix: show the db extra hint for reflex db commands

Closes #7259. On an install without the db extra, reflex db init (and the other db subcommands) crashed with a raw traceback instead of the guided message rx.Model already gives. The db command group now checks for sqlalchemy, sqlmodel and alembic up front and exits 1 with the same pip install reflex[db] hint. Added a parametrized CLI test for init, migrate and makemigrations that fails before the change, and checked it by hand on a bare install.