BE-003: Add integration test for dedup sync

Track: 1 — MVP
Section: 1.1 Device Sync — Deduplication
Status: [ ] Todo
Repo: backend-api
Branch: feature/BE-003-dedup-integration-test

Done When

Test in tests/ sends 50 metrics, then sends same 50 again. Asserts DB has exactly 50 rows. Test passes in CI

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