Promotion engine

- Declarative rules store with multiple rule types: `AUTO_GIFT`, `BUY_X_GET_Y`, and more. - Conditions supported: cart subtotal, product present, cart quantity, customer role, date ranges, coupon presence, category counts; combinators: AND / OR / NOT. - Match strategies: `first` and `best` with rule priorities. - Auto-add free gift product to cart (supports simple & variable products) with protections for stock and purchasability. - Marks promo-added items with metadata: `ssit_gift`, `ssit_rule_id`, and `ssit_unique` to avoid merging with customer-selected products and to ensure correct UI behavior. - UI decorations (badge, "Free" price, promotion meta) are shown only for genuine promo gift lines and only when the gift accompanies other non-gift items. - Admin area: rules CRUD, rule naming, Settings page, Dashboard with basic stats.