Implement the scraper history endpoint that returns past scrape run
records from the scraper_runs collection. Features include:
- GET /api/admin/scraper/history protected by requireAuth + requireAdmin
- Pagination via limit (1-100, default 30) and offset (default 0) params
- Input validation returning 400 for invalid limit/offset values
- Results sorted by startedAt descending (newest first)
- Graceful error handling returning 503 on database failures
- 20 new tests covering auth, pagination, validation, and error cases
