db: add nullable Viewer.Udid with partial unique index

Backstop for /tool/signup idempotency: signup-created viewers carry
the client's UDID (the AES key for that client's wire traffic);
admin-imported viewers stay null. Partial unique index allows the
column to coexist with pre-existing null rows.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

SVSim.Database/Models/Viewer.cs

@@ -8,6 +8,7 @@ namespace SVSim.Database.Models;
 /// A user within the game system.
 /// </summary>
 [Index(nameof(ShortUdid))]
+[Index(nameof(Udid), IsUnique = true)]
 public class Viewer : BaseEntity<long>
 {
     [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
@@ -17,11 +18,18 @@ public class Viewer : BaseEntity<long>
     /// This user's name displayed in game.
     /// </summary>
     public string DisplayName { get; set; } = String.Empty;
-    
+
     /// <summary>
     /// This user's short identifier.
     /// </summary>
     public long ShortUdid { get; set; }
+
+    /// <summary>
+    /// The client's full UDID (AES key for the wire protocol). Set when the viewer is created
+    /// via <c>/tool/signup</c>; null for viewers created via the admin Steam-import path. Unique
+    /// when present — the partial filter is declared in the migration.
+    /// </summary>
+    public Guid? Udid { get; set; }
     
     public DateTime LastLogin { get; set; }