Mark milestone as complete

Mark a milestone as completed. This action automatically completes all associated tasks and records the completion timestamp and user.

Use Cases:

• Track milestone completion in workflow
• Trigger downstream automations
• Auto-complete related tasks
• Record completion audit trail

What Happens:

1. Milestone is marked complete with current timestamp
2. Completion user is recorded
3. All associated tasks are automatically completed
4. Webhooks are triggered for automation
5. Progress metrics are updated

Idempotent: Safe to call multiple times - if already complete, returns success without changes.

Example Response:

{
  "id": "milestone-uuid",
  "name": "Inspection Completed",
  "completed": true,
  "completedAt": "2025-11-08T10:30:00Z",
  "completedByUserId": "user-uuid",
  "tasksProgressPercentage": 100
}