[AGENT] Fix attendee display for personal desktop recordings

[BASIC-BIT]

[AGENT]

Summary

Personal desktop recordings can show attendees as Unknown in the portal even though transcript speaker labels correctly show Me and System/Other.

Current behavior

• Desktop capture labels owner microphone audio as Me.
• Desktop capture labels system output as System/Other.
• Personal upload processing writes those labels into transcript segments.
• Completed personal meeting records currently persist an empty participant list and no attendee fallback.
• The meeting detail UI receives no attendee data and renders Unknown.

Expected behavior

New personal desktop recordings should persist attendee/participant metadata that lets the portal render a meaningful owner/uploader attendee label instead of Unknown.

Scope

• Fix new recordings only. No production backfill is required.
• Preserve the existing transcript speaker labels.
• Keep system output as a transcript speaker/source label, not necessarily a human attendee.
• Add tests around personal upload processing and meeting detail attendee resolution.

Acceptance criteria

1. A new desktop personal recording renders a non-Unknown attendee label in meeting detail.
2. Transcript speaker labels remain Me and System/Other.
3. Personal upload processing persists an attendee or participant shape consistent with existing meeting detail APIs.
4. Tests cover the personal desktop recording case.