fix: dashboard metrics, chart endpoints, varied simulate

[Goodnight77]

Fixes dashboard data issues reported from the demo.

• Add missing /metrics/trend and /metrics/faults-per-machine endpoints (charts showed 'No data')
• Diagnosis latency now measures fault->work-order-created (the AI step), not fault->dispatched which included the human approval wait and inflated MTTR (was ~140 min, now ~1 min) -> ROI calculator no longer shows 0
• Simulate button rotates 7 varied faults (E/J/C/P/A across machines) instead of only E-471
• Tests for trend/faults-per-machine aggregation