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/ViewerSerialCodeRedemption.cs

@@ -0,0 +1,13 @@
+namespace SVSim.Database.Models;
+
+/// <summary>
+/// One row per (viewer, code) redemption. Composite PK on <c>(ViewerId, SerialCodeId)</c>
+/// enforces the single-use-per-viewer guarantee at the DB layer; the controller catches
+/// the unique-constraint violation as a race-condition backstop.
+/// </summary>
+public class ViewerSerialCodeRedemption
+{
+    public long ViewerId { get; set; }
+    public int SerialCodeId { get; set; }
+    public DateTime RedeemedAt { get; set; }
+}