feat(serial-code): add SerialCode + SerialCodeReward + ViewerSerialCodeRedemption entities

Three new EF entities for /campaign/regist_serial_code: SerialCodeEntry (code, message,
window, enabled flag), SerialCodeRewardEntry (FK child, per-slot reward), and
ViewerSerialCodeRedemption (composite-PK redemption record). Registered in SVSimDbContext
with unique index on Code and cascade FK constraints. 3/3 persistence tests pass.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

SVSim.Database/Models/SerialCodeRewardEntry.cs

@@ -0,0 +1,24 @@
+using SVSim.Database.Common;
+
+namespace SVSim.Database.Models;
+
+/// <summary>
+/// One reward slot belonging to a <see cref="SerialCodeEntry"/>. On redemption each row
+/// becomes one <see cref="ViewerPresent"/> in the player's gift inbox.
+/// </summary>
+public class SerialCodeRewardEntry : BaseEntity<int>
+{
+    public int SerialCodeId { get; set; }
+
+    /// <summary>0-based ordering within the code's rewards.</summary>
+    public int Slot { get; set; }
+
+    /// <summary>UserGoodsType cast to int (matches the wire convention used elsewhere).</summary>
+    public int RewardType { get; set; }
+
+    /// <summary>Detail id for the goods. 0 for wallet currencies.</summary>
+    public long RewardDetailId { get; set; }
+
+    /// <summary>Positive integer count.</summary>
+    public int RewardCount { get; set; }
+}