curl -X POST '/subscription/subscribe' \
  -H 'Authorization: Bearer YOUR_ACCESS_TOKEN' \
  -H 'Content-Type: application/json' \
  -d '{
    "tier_id": "tier_pro"
  }'

{
  "success": true,
  "message": "Subscription activated",
  "data": {
    "tier_id": "tier_pro",
    "tier_name": "Pro",
    "status": "active",
    "auto_renew": true,
    "current_period_start": "2026-04-01T00:00:00Z",
    "current_period_end": "2026-05-01T00:00:00Z"
  }
}