BE-016: Track backfill progress in user_devices.meta

Track: 1 — MVP
Section: 1.5 Historical Backfill
Status: [ ] Todo
Repo: backend-api
Branch: feature/BE-016-backfill-progress-tracking

Done When

During backfill, meta shows { backfill_status: "in_progress", backfill_start: "...", points_received: N }. After completion: { backfill_status: "complete" }

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