perf(database): drop redundant entries_user_status_changed_idx

The index entries_user_status_changed_idx(user_id, status, changed_at)
is a strict prefix of entries_user_status_changed_published_idx(user_id,
status, changed_at, published_at), which serves every query the shorter
index could, including the history page and changed_at pagination.

The planner already prefers the four-column index when both are present,
and after the drop those queries fall back to it with an identical plan.
Removing the redundant index reclaims roughly 10% of the entries index
space (~11 MB per 200k entries) with no query regression.