perf(mailqueue): cover amq_latest_send in the affecteduser index

[solracsf]

Summary

The email digest cron (BackgroundJob\EmailNotification, which runs on every cron tick) calls
MailQueueHandler::getAffectedUsers():

SELECT amq_affecteduser, MIN(amq_latest_send)
  FROM activity_mq
  WHERE amq_latest_send < ?
  GROUP BY amq_affecteduser
  ORDER BY MIN(amq_latest_send) ASC

The amp_user index only contained amq_affecteduser, so MIN(amq_latest_send) could not be served from the index. MariaDB/MySQL therefore scanned the entire queue and built a temporary table to resolve the GROUP BY on every run..

This PR widens that index to (amq_affecteduser, amq_latest_send), which lets the engine resolve the aggregate with a loose index scan (Using index for group-by), reading one entry per user instead of the whole queue. amp_user is a strict prefix of the new index and becomes redundant, so it is dropped (no net change in index count).

MariaDB 10.11, activity_mq seeded with a ~60k-row backlog across 9 users:

	index	Handler_read_next (index rows walked)	plan
before	amp_user	60,108	full index scan + Using temporary; Using filesort
after	amp_user_send	0 (18 seeks)	Using index for group-by

The result set is identical before and after (verified with FORCE INDEX vs IGNORE INDEX).

[miaulalala]

/backport to stable34

[miaulalala]

/backport to stable33

[miaulalala]

backport to stable32

[miaulalala]

/backport to stable32