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/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");
                 });