docs: expose followers order literal

[subzeroid]

Summary

• Annotate follower ordering helpers with Optional[FOLLOWERS_ORDER].
• Document FOLLOWERS_ORDER = Literal["date_followed_latest", "date_followed_earliest"] in the User usage guide.
• Cover signatures and docs with public literal regression tests.

Test plan

• .venv/bin/ruff check .
• .venv/bin/mkdocs build --strict
• .venv/bin/python -m pytest tests/regression -q