feat: add User model and Alembic migration for users table

tests/test_user_model.py

@@ -0,0 +1,48 @@
+import pytest
+from sqlalchemy import create_engine
+from sqlalchemy.orm import sessionmaker
+
+from app.core.database import Base
+from app.modules.auth.models import User
+
+
+@pytest.fixture
+def db():
+    engine = create_engine("sqlite:///:memory:", connect_args={"check_same_thread": False})
+    Base.metadata.create_all(bind=engine)
+    Session = sessionmaker(bind=engine)
+    session = Session()
+    yield session
+    session.close()
+    Base.metadata.drop_all(bind=engine)
+
+
+def test_create_user(db):
+    user = User(username="testuser", full_name="Test User", pw_hash="hashed")
+    db.add(user)
+    db.commit()
+    db.refresh(user)
+    assert user.id is not None
+    assert user.username == "testuser"
+    assert user.is_active is True
+    assert user.is_admin is False
+    assert user.created_at is not None
+
+
+def test_user_defaults(db):
+    user = User(username="alice", full_name="Alice")
+    db.add(user)
+    db.commit()
+    db.refresh(user)
+    assert user.pw_hash is None
+    assert user.email is None
+    assert user.department is None
+    assert user.last_login is None
+
+
+def test_username_must_be_unique(db):
+    db.add(User(username="dup", full_name="First"))
+    db.commit()
+    db.add(User(username="dup", full_name="Second"))
+    with pytest.raises(Exception):
+        db.commit()