BE-021: Implement readiness score calculation

Track: 1 — MVP
Section: 1.7 Health Dashboard API
Status: [ ] Todo
Repo: backend-api
Branch: feature/BE-021-readiness-score

Done When

GET /health/dashboard returns readiness_score: 0-100 based on last night sleep + resting HR + HRV. Formula documented in code comment. Returns null with reason when <3 days data

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