feat: configurable KPI targets — refund rate, conversion rate thresholds

Add per-conference configurable targets beyond budgets:
- `acceptable_refund_rate` (default 5%)
- `target_conversion_rate` (cart → order)
- `target_checkin_rate`
- Show warning/success indicators when actuals breach thresholds
- Refund gauge zones should use the configured threshold

Depends on: #46