BE-015: Add is_backfill: bool flag to sync request

Track: 1 — MVP
Section: 1.5 Historical Backfill
Status: [ ] Todo
Repo: backend-api
Branch: feature/BE-015-backfill-flag

Done When

When is_backfill: true, skip real-time alerts (don't fire abnormal HR notification for old data). Test: backfill with HR=200 from last week → no push notification sent

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