Move the users table into the shared persistence engine so auth
matches the pattern of threads_meta, runs, run_events, and feedback —
one engine, one session factory, one schema init codepath.
New files
---------
- persistence/user/__init__.py, persistence/user/model.py: UserRow
ORM class with partial unique index on (oauth_provider, oauth_id)
- Registered in persistence/models/__init__.py so
Base.metadata.create_all() picks it up
Modified
--------
- auth/repositories/sqlite.py: rewritten as async SQLAlchemy,
identical constructor pattern to the other four repositories
(def __init__(self, session_factory) + self._sf = session_factory)
- auth/config.py: drop users_db_path field — storage is configured
through config.database like every other table
- deps.py/get_local_provider: construct SQLiteUserRepository with
the shared session factory, fail fast if engine is not initialised
- tests/test_auth.py: rewrite test_sqlite_round_trip_new_fields to
use the shared engine (init_engine + close_engine in a tempdir)
- tests/test_auth_type_system.py: add per-test autouse fixture that
spins up a scratch engine and resets deps._cached_* singletons
ceeccabc98