messages(message_id, user_id, sent_at)

For the calendar week beginning 2026-09-07 (seven days, up to and including 2026-09-13), build a histogram of how busy users were: how many users sent exactly 1 message, how many sent exactly 2, and so on.

Users who sent nothing that week do not appear. Return messages_sent and user_count, ordered by messages_sent.

Two levels of grouping: count per user first, then count the users at each count.

Tables

messages

message_id  user_id  sent_at
----------  -------  -------------------
1           10       2026-09-07 09:00:00
2           10       2026-09-09 10:00:00
3           11       2026-09-08 11:00:00
4           12       2026-09-10 12:00:00
5           12       2026-09-11 13:00:00
6           12       2026-09-13 23:59:59
7           13       2026-09-06 23:59:59
8           14       2026-09-14 00:00:00

Expected result

messages_sent  user_count
-------------  ----------
1              1
2              1
3              1