Admin UI: let admins toggle delivery channel per team member

[franrob-projects]

The team_members table now has a delivery_channel column ('slack' or 'teams'), but there's no UI for admins to change it.

Add a dropdown or toggle in the ComplianceMatrix (src/components/admin/ComplianceMatrix.tsx) that lets admins switch each team member between Slack and Teams delivery.

Requires:

• A PATCH endpoint or update to the existing team API
• A small UI element per row in the compliance matrix
• Optimistic update for a snappy UX