fix(guild): populate leader_name in search results (matches prod capture)

Add IViewerRepository.LoadDisplayNamesAsync (batch AsNoTracking select),
extend GuildSearchEntry with LeaderName, enrich GuildService.SearchAsync
with a leader-id batch lookup, and thread the name through the controller's
ToDetailDto call. Adds a leader_name assertion to GuildSearchFlowTests.

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

SVSim.Database/Repositories/Viewer/IViewerRepository.cs

@@ -33,4 +33,10 @@ public interface IViewerRepository
 
     /// <summary>Clears Viewer.GuildId to null. No-op if the viewer does not exist.</summary>
     Task ClearGuildIdAsync(long viewerId, CancellationToken ct = default);
+
+    /// <summary>
+    /// Batch-loads <c>DisplayName</c> for a set of viewer ids. Returns a dictionary keyed by viewer id;
+    /// ids with no matching row are absent from the result. Read-only (AsNoTracking).
+    /// </summary>
+    Task<Dictionary<long, string>> LoadDisplayNamesAsync(IReadOnlyCollection<long> viewerIds, CancellationToken ct = default);
 }