Upload Message Attachments

Attach files to a not-yet-sent reply. Both verticals: gated exactly like every other write on this ticket (_require_kind_grant_or_assignee -- a b2c CS agent and a b2g account manager both reach this the same way, through the SAME grant check every other message action here uses).

Refuses attaching to anything but a draft, outbound message: a sent message has already left, and attaching to it now would store a file the customer never received while the thread implies otherwise -- the same "never claim a file arrived when it did not" floor AttachmentRejected/AttachmentTooLarge enforce per-file, applied at the message level instead.

EVERY FILE VALIDATED BEFORE ANY WRITE (mirrors issues/routes.py's attach_to_intake): a batch that fails on file 2 of 3 stores nothing, rather than a half-attached reply.