feat: Story 023 — Data Synchronization (Offline Check-in)

[iagocavalcante]

Summary

• Add device_assignments table linking devices to events, and device_id FK on check_ins for tracking offline origins
• Add Pretex.Sync context with manifest download (full + incremental) and offline check-in upload with earliest-timestamp-wins conflict resolution
• Add DeviceAuth plug and JSON API endpoints: GET /api/sync/manifest, POST /api/sync/checkins
• Add device-to-event assignment UI in admin device management page

Test plan

• Run mix ecto.migrate to apply new migration
• Run mix test — verify all new tests pass (device assignments, sync context, device auth plug, sync controller)
• Provision a device, assign it to an event, and verify GET /api/sync/manifest returns attendee data
• Upload offline check-ins via POST /api/sync/checkins and verify conflict resolution (earliest timestamp wins)
• Test incremental sync with ?since= parameter returns only changes
• Verify admin UI shows event assignment tags with add/remove functionality

Closes #23

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