fix: add serializer removed during rebase

- Add CourseHomeTabSerializer to v1/serializers/home.py (was imported by
  v3/views/home.py but never defined, causing ImportError on startup)
- Export it from v1/serializers/__init__.py
- Remove stale HomePageCoursesView from v1/views/__init__.py (class was
  dropped by revert commit during rebase but export remained)
- Remove unused UserFactory and CourseOverviewFactory imports from
  v1/views/tests/test_home.py (ruff F401 violations from rebase conflict)