T2-005: Upload validation and error reporting

Track: 2 — UI Revamp + AI Analytics
Section: Backend API -- Data Upload & Storage
Status: [ ] Todo
Repo: backend-api
Branch: feature/T2-005-upload-validation

Done When

If CSV has bad rows, return { processed: 45, errors: [{row: 12, field: "heart_rate", error: "value 500 out of range"}] }. Partial success allowed

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