feat: complete auth/profile username migration and runtime safeguards

backend/tests/unit/v1/auth/test_auth_models.py

@@ -14,7 +14,14 @@ from v1.auth.schemas import (
 
 def test_signup_requires_valid_email() -> None:
     with pytest.raises(ValidationError):
-        SignupRequest(email="not-an-email", password="secret123")
+        SignupRequest(username="demo", email="not-an-email", password="secret123")
+
+
+def test_signup_requires_username() -> None:
+    with pytest.raises(ValidationError):
+        SignupRequest.model_validate(
+            {"email": "user@example.com", "password": "secret123"}
+        )
 
 
 def test_login_requires_valid_email() -> None: