feat: implement DORA metric calculation engine (DF, LT, MTTR, CFR) [RR-59]

[devin-ai-integration]

Summary

Adds a new dora feature module with a DoraMetricsService that transforms raw pipeline events (deploys, commits, incidents) into the four DORA metrics:

• Deployment Frequency (DF): Counts successful deploys over a period, computes deploys/day
• Lead Time (LT): Measures time from earliest commit to its associated successful deploy (average + median)
• Mean Time to Restore (MTTR): Measures incident open→resolve duration for resolved incidents (average + median)
• Change Failure Rate (CFR): Ratio of failed deploys to total deploys

Each metric is rated elite | high | medium | low based on hardcoded thresholds. A calculateAll() convenience method computes all four at once.

This is calculation logic only — no API integration, no UI, no route. The service is registered as providedIn: 'root' but has no consumers yet.

36 Vitest unit tests cover all calculations, rating thresholds, edge cases (empty inputs, zero periods, all-failed/all-success), and median computation for odd/even counts.

Review & Testing Checklist for Human

• Verify rating thresholds match your DORA benchmark expectations. The thresholds are hardcoded in private rate* methods (e.g., elite DF ≥ 1/day, elite LT ≤ 24h, elite MTTR ≤ 1h, elite CFR ≤ 5%). These are not configurable — confirm they align with team standards.
• Confirm lead time methodology. Lead time is measured from the earliest commit linked to a deploy. If your definition differs (e.g., PR merge time, or per-commit rather than per-deploy), this needs adjustment.
• Assess scope sufficiency for RR-59. This PR delivers pure calculation logic with no data source integration. Confirm whether the ticket expects only the engine, or also an API/UI layer.
• Note: no input validation. Timestamps are parsed via new Date(string) with no format validation; invalid timestamps would produce NaN results silently. The service field on events is present in the model but not used for filtering — all events are processed together.

Notes

• Tests instantiate the service directly (new DoraMetricsService()) rather than through Angular TestBed, since the service has zero Angular DI dependencies. This avoids the pre-existing zone.js missing-dependency issue in the test setup.
• The service field on event models is available for future per-service filtering but is not currently used in calculations.

Link to Devin session: https://app.devin.ai/sessions/04b630c9eed34b9281b656b35421e447

[devin-ai-integration]

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