BE-004: Add field-level validation to POST /health/metrics/bulk

Track: 1 — MVP
Section: 1.2 Device Sync — Bulk Validation
Status: [ ] Todo
Repo: backend-api
Branch: feature/BE-004-bulk-field-validation

Done When

HR outside 20-300 → rejected. SpO2 outside 0-100 → rejected. Steps < 0 → rejected. Response: 422 with array of { field, value, error } per invalid item

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