BE-032: Return empty array (not error) when insufficient data

Track: 1 — MVP
Section: 1.10 Health Insights API
Status: [ ] Todo
Repo: backend-api
Branch: feature/BE-032-empty-insights-graceful

Done When

User with 0 days data → GET /health/insights returns { patterns: [], message: "Need at least 7 days of data" }. 200 status

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