perf(db): index mailbox list sort and recipient lookup

[ChristophWurst]

The mailbox list query (mailbox_id + flag_deleted, ORDER BY sent_at) had no covering index and fell back to a filesort; the standalone sent_at index was dropped in 2022 and never restored. mail_recipients.local_message_id had a foreign key but no backing index, so lookups and deletes scanned the whole table on Postgres and SQLite.

Register both via AddMissingIndicesEvent so existing installations add them out-of-band through occ db:add-missing-indices, and add them to the creating migrations so new installations get them up front.

Assisted-by: Claude:claude-opus-4-8

How to test

EXPLAIN SELECT m.id, m.sent_at FROM oc_mail_messages m
        WHERE m.mailbox_id = 9658 AND m.flag_deleted = 0
        ORDER BY m.sent_at DESC LIMIT 20;

EXPLAIN SELECT m.id, m.sent_at
        FROM oc_mail_messages m
        WHERE m.mailbox_id = 8464          
          AND m.flag_deleted = 0
          AND m.sent_at < 1429201770           
        ORDER BY m.sent_at DESC
        LIMIT 20;

^ replace IDs with something existent of your local db.

Before:

1. 1|SIMPLE|m|ref|mail_messages_mb_id_uid_uidx,mail_msg_thrd_root_snt_idx,mail_msg_by_remote_id_idx|mail_messages_mb_id_uid_uidx|4|const|1|Using where; Using filesort
2. 1|SIMPLE|m|ref|mail_messages_mb_id_uid_uidx,mail_msg_thrd_root_snt_idx,mail_msg_by_remote_id_idx|mail_messages_mb_id_uid_uidx|4|const|525|Using where; Using filesort
   After
3. 1|SIMPLE|m|ref|mail_messages_mb_id_uid_uidx,mail_msg_thrd_root_snt_idx,mail_msg_by_remote_id_idx,mail_msg_mb_del_snt_idx|mail_msg_mb_del_snt_idx|6|const,const|1|Using where; Using index
4. 1|SIMPLE|m|range|mail_messages_mb_id_uid_uidx,mail_msg_thrd_root_snt_idx,mail_msg_by_remote_id_idx,mail_msg_mb_del_snt_idx|mail_msg_mb_del_snt_idx|10||51|Using where; Using index

-> no more filesort

🤖 AI (if applicable)

• The content of this PR was partly or fully generated using AI

[ChristophWurst]

@CodeRabbit ai review and evaluate the before/after of the PR description against the code base