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/Migrations/20260527175337_AddViewerUdid.cs

@@ -0,0 +1,40 @@
+using System;
+using Microsoft.EntityFrameworkCore.Migrations;
+
+#nullable disable
+
+namespace SVSim.Database.Migrations
+{
+    /// <inheritdoc />
+    public partial class AddViewerUdid : Migration
+    {
+        /// <inheritdoc />
+        protected override void Up(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.AddColumn<Guid>(
+                name: "Udid",
+                table: "Viewers",
+                type: "uuid",
+                nullable: true);
+
+            migrationBuilder.CreateIndex(
+                name: "IX_Viewers_Udid",
+                table: "Viewers",
+                column: "Udid",
+                unique: true,
+                filter: "\"Udid\" IS NOT NULL");
+        }
+
+        /// <inheritdoc />
+        protected override void Down(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.DropIndex(
+                name: "IX_Viewers_Udid",
+                table: "Viewers");
+
+            migrationBuilder.DropColumn(
+                name: "Udid",
+                table: "Viewers");
+        }
+    }
+}

SVSim.Database/Migrations/SVSimDbContextModelSnapshot.cs

@@ -2052,10 +2052,16 @@ namespace SVSim.Database.Migrations
 
                     NpgsqlPropertyBuilderExtensions.UseSequence(b.Property<long>("ShortUdid"), "ShortUdidSequence");
 
+                    b.Property<Guid?>("Udid")
+                        .HasColumnType("uuid");
+
                     b.HasKey("Id");
 
                     b.HasIndex("ShortUdid");
 
+                    b.HasIndex("Udid")
+                        .IsUnique();
+
                     b.ToTable("Viewers");
                 });