Add item tracking migrations

Distribution/Models/StockBalance.cs

@@ -13,6 +13,16 @@ namespace Distribution.Models
         public Guid LocationId { get; set; }
         public LocationType LocationType { get; set; }
 
+        /// <summary>Null for items that don't track batches/expiry — no behavior change for existing items.</summary>
+        public Guid? FK_ItemBatchId { get; set; }
+        [ForeignKey("FK_ItemBatchId")]
+        public ItemBatch? ItemBatch { get; set; }
+
+        /// <summary>Null for items that don't track serial numbers.</summary>
+        public Guid? FK_ItemSerialNumberId { get; set; }
+        [ForeignKey("FK_ItemSerialNumberId")]
+        public ItemSerialNumber? ItemSerialNumber { get; set; }
+
         public decimal Quantity { get; set; }
         public DateTime UpdatedAt { get; set; } = DateTime.Now;
     }