BE-010: Add sync_count column to user_devices — increment on each sync

Track: 1 — MVP
Section: 1.3 Device Sync — Status Tracking
Status: [ ] Todo
Repo: backend-api
Branch: feature/BE-010-sync-count-column

Done When

Migration adds column default 0. Each POST /devices/{id}/sync increments by 1. GET /devices/{id} returns sync_count

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