Get Staff Group Available Slots

Get available time slots for a staff group on a given day.

This endpoint is authenticated using a service token (Bearer token in Authorization header).

This endpoint:

1. Resolves the staff group from case_id (or defaults to first Office group)
2. Gets the members of the staff group
3. Fetches their availability schedules for that day
4. Fetches their calendar events (only for calendars used for availability)
5. Overlays them with the group's meeting configuration (respecting timezones)
6. Returns time slots where at least one user is available (no calendar events overlapping)

• date: The date to check availability for (ISO format, e.g., '2025-10-15')
• case_id: Optional ID of case to get assigned staff group (defaults to first Office group)

Returns: List of available time slots with the user IDs that are available for each slot.