Update API endpoints and schemas to use the unified Order model:
- Update Letzshop order schemas with OrderItem support
- Update API responses to use new field names (external_*, status, etc.)
- Update confirm/reject endpoints to use OrderItem.external_item_id
- Update Letzshop order service for unified Order model queries
- Update documentation to reflect completed implementation
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>