Sync stock status on restock and expose live stock on getproductbyvariant

Two gaps found while auditing the order/stock flow:

- Receiving stock via an approved stock request only updated
  products.productstatus (a global per-product flag). A location
  flagged outofstock by CreateOrder never got reactivated, since
  nothing touched productlocations.status on the way back in.
  CreateProductStock now reactivates the specific
  (tenant, location, product) row to "available" for every "in"
  entry, the counterpart to how it gets flagged out.

- getproductbyvariant returned no stock info at all, so the app could
  only find out a product was unavailable from the 409 at order time.
  It now accepts an optional locationid and, when passed, returns
  live productstock (same SUM(in)-SUM(out) formula the order check
  uses) and locationstatus per product. Omitting locationid keeps the
  old response shape.

Added MOBILE_ORDER_VERIFICATION.md as a handoff doc for the mobile
team covering the expected request/response shapes and how to verify
their integration.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

models/product.go

@@ -83,6 +83,13 @@ type Products struct {
 	Othercost         float64 `json:"othercost,omitempty"`
 	Approve           int     `json:"approve"`
 	Productstatus     string  `json:"productstatus" `
+	// Populated only by queries scoped to a specific location (e.g.
+	// GetProductByVariant when locationid is passed): Productstock becomes
+	// the live SUM(in)-SUM(out) balance from productstocks — the same
+	// formula CreateOrder's stock check uses — and Locationstatus mirrors
+	// productlocations.status ("outofstock"/"available") for that store.
+	// Left at zero values for callers that don't scope to a location.
+	Locationstatus string `json:"locationstatus,omitempty" gorm:"->"`
 	// Status string `json:"status" gorm:"default:InActive"`
 	// Status string `json:"status" gorm:"-"`
 }
@@ -306,6 +313,15 @@ type Productlocations struct {
 	Status            string  `json:"status"`
 }
 
+// ProductLocationRef identifies a single (tenant, location, product) row in
+// productlocations — used to reactivate it after new stock arrives, the
+// per-location counterpart to CreateOrder's outofstock flag.
+type ProductLocationRef struct {
+	Tenantid   int
+	Locationid int
+	Productid  int
+}
+
 type ProductSubcategory struct {
 	Subcatid   int       `json:"subcatid"`
 	Categoryid int       `json:"categoryid"`