Make sales order inventory updates atomic

[gina941111-debug]

Summary

• create sales orders, reserve inventory, and write stock movements in one SQLite transaction
• use conditional inventory updates under an immediate transaction to prevent concurrent overselling
• preserve cancelled orders and restore inventory exactly once
• safely re-reserve inventory when cancelled orders are reactivated
• route both Streamlit and AI order creation through the shared backend service
• generate collision-resistant AI order IDs

Validation

• 49 focused transaction and approval tests passed
• 332 full project tests passed
• Python compilation and git diff checks passed

Notes

• Existing historical records are not automatically reconciled by this change
• Untracked local documentation and system files are not included