Add arun_migrations, the awaitable form of run_migrations

[jph00]

arun_migrations(db_path, migrations_dir, verbose=False) runs a custom backend's hooks on the calling event loop, so an async application can await migrations at startup instead of blocking its loop while run_migrations spins up a thread and a private loop for the same work. The SQLite path runs run_migrations in a worker thread. run_migrations is unchanged.

The custom-backend test now runs the migrations through arun_migrations and then checks that run_migrations finds the database up to date. The README and docsrc/custom_backends.qmd mention the new function.