Mark a follow-up as lost (manager only).

Sets followup_v2 status to 'lost' with the provided dismissal reason and archives the conversation.

This endpoint is accessible by managers only (org:admin role).

Valid dismissal_reason values:

• "too_expensive"
• "chose_competitor"
• "scope_disagreement"
• "equipment_preference"
• "approval_issue"
• "other"
• "other: <custom text>" (custom reason after "other: ")

Args: conversation_id: UUID of the conversation to find the followup dismissal_reason: Reason for marking as lost organization_id: Organization ID from auth token

Returns: Success response with followup_id and dismissal_reason

Raises: 400: If dismissal_reason is invalid 404: If followup not found in organization 500: If database operation fails