Import Discord call messages as call evidence

The situation

Discord private calls can carry a participant roster and an optional end timestamp. Msgvault's guild-bot source preserves raw message JSON but does not fetch private call history.

What works today, and why it is not enough

The call-history work in #1406 accepts normalized observations. Turning an archived Discord CALL message or a supplied export into that shape still requires manual mapping, with room to mistake mentions or an ended call for proof of an answer.

Ask

Accept native Discord CALL messages, existing archived Discord JSON by message ID, and individual messages from supplied DiscordChatExporter exports through the call-history import path. Keep unknown fields unknown and preserve richer native evidence when a lossy export is imported too.

Live Discord acquisition and user-token automation are outside this request.