feat(tutorial): add /tutorial/gift_top with hardcoded starter present list

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

SVSim.Database/Models/ViewerClaimedTutorialGift.cs

@@ -0,0 +1,14 @@
+namespace SVSim.Database.Models;
+
+/// <summary>
+/// Records that a viewer has claimed a specific tutorial gift present_id. Composite key
+/// (ViewerId, PresentId) — viewer can't claim the same present twice.
+/// </summary>
+public class ViewerClaimedTutorialGift
+{
+    public long ViewerId { get; set; }
+    public string PresentId { get; set; } = string.Empty;
+    public DateTime ClaimedAt { get; set; }
+
+    public Viewer Viewer { get; set; } = null!;
+}