feat: complete auth/profile username migration and runtime safeguards

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

@@ -16,7 +16,6 @@ from v1.profile.service import ProfileService
 def _create_mock_profile(
     user_id: UUID = UUID("00000000-0000-0000-0000-000000000001"),
     username: str = "demo",
-    display_name: str | None = "Demo User",
     avatar_url: str | None = None,
     bio: str | None = None,
 ) -> Profile:
@@ -24,7 +23,6 @@ def _create_mock_profile(
     profile = MagicMock(spec=Profile)
     profile.id = user_id
     profile.username = username
-    profile.display_name = display_name
     profile.avatar_url = avatar_url
     profile.bio = bio
     return profile
@@ -115,9 +113,9 @@ async def test_update_me_updates_fields(mock_session: AsyncMock) -> None:
         current_user=user,
     )
 
-    result = await service.update_me(ProfileUpdateRequest(display_name="Updated"))
+    result = await service.update_me(ProfileUpdateRequest(username="updated"))
 
-    assert result.display_name == "Updated"
+    assert result.username == "updated"
     mock_session.commit.assert_awaited_once()
 
 
@@ -134,7 +132,7 @@ async def test_update_me_no_fields_raises_400(mock_session: AsyncMock) -> None:
 
     # Create a request with all None values by bypassing validation
     update = MagicMock(spec=ProfileUpdateRequest)
-    update.display_name = None
+    update.username = None
     update.avatar_url = None
     update.bio = None