Add item tracking migrations

Distribution/Models/ItemSerialNumber.cs

@@ -0,0 +1,26 @@
+using System.ComponentModel.DataAnnotations.Schema;
+
+namespace Distribution.Models
+{
+    /// <summary>One row per physical serial-tracked unit. Quantity is always 1.</summary>
+    public class ItemSerialNumber : Template
+    {
+        public Guid FK_ItemId { get; set; }
+        [ForeignKey("FK_ItemId")]
+        public Item Item { get; set; }
+
+        public string SerialNumber { get; set; }
+        public SerialStatus Status { get; set; } = SerialStatus.InStock;
+
+        public Guid? CurrentLocationId { get; set; }
+        public LocationType? CurrentLocationType { get; set; }
+
+        public DateTime ReceivedAt { get; set; }
+
+        /// <summary>Snapshotted at receipt when Item.AllowCustomWarrantyPeriod is true; otherwise null and the system default applies when the warranty starts.</summary>
+        public int? WarrantyPeriodDaysOverride { get; set; }
+
+        /// <summary>Set when the serial is sold/delivered: sale date + (override ?? system default). Null while in stock or if Item.HasWarranty is false.</summary>
+        public DateTime? WarrantyExpiresAt { get; set; }
+    }
+}