[Test]: Expand mockdata with realistic data volumes

## Description
Current mockdata has minimal rows per domain.

## Location
- **Files**: `mockdata/`

## Current State
Only 1-4 rows per domain.

## Expected State
Realistic data volumes for testing performance.

## Acceptance Criteria
- [ ] Add larger test datasets
- [ ] Include edge cases
- [ ] Document test data
- [ ] Tests pass