BE-011: Fix POST /devices upsert — same device_identifier updates existing row

Track: 1 — MVP
Section: 1.4 Device Registration
Status: [x] Done Repo: backend-api Branch: feature/BE-001-health-metrics-unique-constraint

Done When

Register device with MAC AA:BB:CC. Register again with same MAC but new firmware_version → existing row updated, no new row created. Returns the updated device

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