ADD: Warehouse inventory management system

- Add WarehouseItem model with bilingual names (FA/EN) and bilingual counting units
- Add WarehouseItemTransaction model for IN/OUT movements with reason and actor
- Add WarehouseItemStock model for denormalized per-(item×warehouse) running balance
- Add warehouseItemService with full CRUD, atomic stock updates via $inc, negative stock guard, and aggregation queries for per-item/per-warehouse/total stock
- Add warehouseItemController and REST routes at /api/warehouse-items
- Add warehouseService, warehouseController, warehouseRoutes for warehouse location CRUD at /api/warehouses
- Register both route groups in routerEndpoints.js

components/warehouseItem/warehouseItemStockModel.js

@@ -0,0 +1,39 @@
+const mongoose = require('mongoose')
+const ObjectID = mongoose.Schema.Types.ObjectId
+
+// Running balance snapshot per (item × warehouse)
+// Updated atomically on each transaction using $inc
+// Acts as a fast-read cache — transactions are the source of truth
+const warehouseItemStockSchema = new mongoose.Schema(
+    {
+        item: {
+            type: ObjectID,
+            ref: 'WarehouseItem',
+            required: true,
+        },
+        warehouse: {
+            type: ObjectID,
+            ref: 'Warehouse',
+            required: true,
+        },
+        quantity: {
+            type: Number,
+            default: 0, // current balance
+        },
+        unit: {
+            type: String,
+        },
+        unitEn: {
+            type: String,
+        },
+    },
+    {
+        timestamps: true,
+    }
+)
+
+// Guarantee one document per (item, warehouse) combination
+warehouseItemStockSchema.index({ item: 1, warehouse: 1 }, { unique: true })
+
+const WarehouseItemStock = mongoose.model('WarehouseItemStock', warehouseItemStockSchema)
+module.exports = WarehouseItemStock