BE-020: Handle empty data gracefully — return nulls, not errors

Track: 1 — MVP
Section: 1.6 Health Summary API
Status: [x] Done Repo: backend-api Branch: feature/BE-001-health-metrics-unique-constraint

Done When

GET /health/summary?date=2020-01-01 (no data) returns 200 with all fields null and has_data: false. Not 404 or 500

Notes

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