feat(shared): extend CreateSessionCommand with showcase metadata

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

src/GmRelay.Shared/Features/Sessions/CreateSession/CreateSessionCommand.cs

@@ -1,3 +1,4 @@
+using GmRelay.Shared.Domain;
 using GmRelay.Shared.Platform;
 
 namespace GmRelay.Shared.Features.Sessions.CreateSession;
@@ -9,4 +10,9 @@ public sealed record CreateSessionCommand(
     string Link,
     IReadOnlyList<DateTimeOffset> ScheduledTimes,
     int? MaxPlayers,
-    string? ImageReference);
+    string? ImageReference,
+    GameSystem? System = null,
+    string? Description = null,
+    string? Format = null,
+    int? DurationMinutes = null,
+    bool IsOneShot = false);