GET /api/v1/external/forms/:formId/stats

{
  "data": {
    "total_submissions": 796,
    "complete_submissions": 342,
    "completion_rate": 42.96,
    "avg_completion_time_seconds": 67,
    "submissions_by_day": [
      { "date": "2026-05-14", "count": 23 },
      { "date": "2026-05-15", "count": 31 }
    ]
  }
}