Pre-Order & Sales Limiter
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Adds variant-level pre-order control to Odoo.

* Auto-Sync Setup — enabling pre-order on a variant auto-activates
"Continue Selling" (sell when out of stock). No manual setup.
* Dynamic Storefront — "Add to Cart" switches to "Pre-Order" with
custom messaging the moment stock hits zero, and reverts automatically.
* Live Quantity Cap — remaining slots (limit minus sold) are enforced
directly in the storefront quantity field.
* Double-Gate Checkout Guard — a server-side check at payment and order
confirmation blocks overselling and rolls back breaches.
* Mixed-Cart Handling — pre-order and in-stock items share one delivery,
flagged clearly for warehouse staff.
* Audit Trail — pre-order notes flow through to invoices and PDFs, with
a one-click filter for reporting.