Reduce IMAP message ID index prefix lengths

[ChristophWurst]

Is your feature request related to a problem? Please describe.

There are three indices on IMAP message IDs. The original data can be 1023B long.

• mail_messages_msgid_idx is 128B
• mail_msg_thrd_root_snt_idx is 64B
• mail_msg_imap_id_idx is 128B

Describe the solution you'd like

Shorten the prefixes to 64B consistently.

Describe alternatives you've considered

Store hashes only, use 32B. Use the hashes for all lookups. Caveat: our operation shave to support that. This is something to do research on.

Additional context

• feat: Add configurable sync window (mail.max_sync_days) to limit database growth #12633