fix(check): merge anonymous resignup viewer into Steam-linked viewer

GameStart already detects the Steam-vs-UDID mismatch produced by
wipe-and-resignup; it now also reclaims the orphan. New
ViewerRepository.MergeAnonymousViewerInto transfers the fresh UDID
from V_new onto V_old in one save (freeing the unique-index slot),
then deletes V_new in a second save. Partial-failure mode is a
benign null-UDID viewer; two rows never contend for the same UDID.
Side benefit: future GetViewerByUdid lookups now short-circuit to
V_old without going through the Steam handler.

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

SVSim.Database/Repositories/Viewer/IViewerRepository.cs

@@ -13,4 +13,11 @@ public interface IViewerRepository
         ulong socialAccountIdentifier, ulong? shortUdid = null);
     Task<Models.Viewer> RegisterAnonymousViewer(Guid udid);
     Task LinkSteamToViewer(long viewerId, ulong steamId);
+
+    /// <summary>
+    /// Merges an anonymous viewer (just created by <c>/tool/signup</c> on a fresh UDID)
+    /// into a target viewer that the Steam ticket resolved to. Transfers the anonymous
+    /// viewer's UDID to the target, then deletes the anonymous viewer.
+    /// </summary>
+    Task MergeAnonymousViewerInto(long anonymousViewerId, long targetViewerId);
 }