BE-027: Sleep insights text generation

Track: 1 — MVP
Section: 1.8 Sleep Analysis API
Status: [ ] Todo
Repo: backend-api
Branch: feature/BE-027-sleep-insights

Done When

Sleep response includes insights: string[] e.g., ["Your deep sleep increased 15% this week", "You went to bed 45 min later than usual"]. At least 1 insight when >3 nights data

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