refactor(auth): migrate user repository to SQLAlchemy ORM

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

backend/packages/harness/deerflow/persistence/user/__init__.py

@@ -0,0 +1,12 @@
+"""User storage subpackage.
+
+Holds the ORM model for the ``users`` table. The concrete repository
+implementation (``SQLiteUserRepository``) lives in the app layer
+(``app.gateway.auth.repositories.sqlite``) because it converts
+between the ORM row and the auth module's pydantic ``User`` class.
+This keeps the harness package free of any dependency on app code.
+"""
+
+from deerflow.persistence.user.model import UserRow
+
+__all__ = ["UserRow"]