feat: complete auth/profile username migration and runtime safeguards

backend/tests/unit/v1/profile/test_schemas.py

@@ -10,7 +10,6 @@ def test_profile_response_maps_fields() -> None:
     response = ProfileResponse(
         id="user-1",
         username="demo",
-        display_name="Demo User",
         avatar_url=None,
         bio=None,
     )
@@ -56,6 +55,11 @@ def test_profile_update_rejects_data_url() -> None:
 
 
 def test_profile_update_accepts_none_avatar_url_with_other_field() -> None:
-    request = ProfileUpdateRequest(display_name="Test", avatar_url=None)
+    request = ProfileUpdateRequest(username="tester", avatar_url=None)
     assert request.avatar_url is None
-    assert request.display_name == "Test"
+    assert request.username == "tester"
+
+
+def test_profile_update_rejects_display_name_field() -> None:
+    with pytest.raises(ValidationError):
+        ProfileUpdateRequest.model_validate({"display_name": "legacy"})