Import archived Slack huddles into call history

The situation

Slack imports already retain huddle messages and their original room payloads. That evidence is absent from the call-history views introduced in #1406.

What works today, and why it is not enough

A normalized observation can be assembled by hand, but that duplicates source-field parsing and participant binding. Room rosters also need conservative handling: their presence does not establish an answered call or joined attendance.

Ask

Add a Slack huddle adapter for supplied payloads and individual archived messages. Reuse existing Slack identifiers and call-history persistence, preserve unknown attendance and duration, and avoid additional Slack authorization or API requests.