Retrieve details of a specific message including all associated evidences.

Important notes:

• Returns 404 if message doesn't exist or user doesn't have access
• Evidence PDFs are signed and cached for 7 days
• If evidences don't exist yet, they will be generated on-demand
• Works with both mTLS and API token authentication
• MT messages return an empty evidences array (MT has no legal evidence)
• For MT, status follows the email delivery lifecycle: SENT (sent) → RECEIVED (delivered to the recipient's mailbox) → OPENED (recipient opened the email or downloaded the attachment); UNDELIVERED on bounce (reversible — a later successful delivery moves it back to RECEIVED); EXPIRED if delivered but never opened within the delay. Note: for MT, RECEIVED means "delivered" (not "downloaded") and is not a terminal status.
• For a scheduled send not yet emitted, status is SCHEDULED and scheduledAt is populated (UTC).