Keep Only Each User's Latest Row

Change-log tables hold every version of a row and you almost always want the current one. Getting this wrong gives you a random version rather than the newest, and it looks fine until someone checks.

Write latest_per_user(df).

df has columns user_id, updated_at (date strings) and plan. There may be several rows per user. Return one row per user: the one with the latest updated_at.