Retrieve a specific evidence document for a message by its status/event type.

Important notes:

• Returns 404 if message doesn't exist, user doesn't have access, or evidence doesn't exist
• Returns 422 (mt-has-no-evidence) if the message is an MT: MT messages never generate evidences
• Only one evidence per status is allowed (returns 404 if multiple found)
• Evidence PDFs are signed and cached for 7 days
• If evidence doesn't exist yet, it will be generated on-demand
• Works with both mTLS and API token authentication