feat: User model and initial Alembic migration

tests/test_models.py

@@ -0,0 +1,12 @@
+import pytest
+from fablednetmon.models.users import User, UserRole
+
+
+def test_user_model_fields():
+    """User model has all required fields."""
+    cols = {c.key for c in User.__table__.columns}
+    assert cols == {"id", "username", "email", "password_hash", "role", "created_at", "is_active"}
+
+
+def test_user_role_enum():
+    assert set(UserRole) == {UserRole.admin, UserRole.operator, UserRole.viewer}