feat(shared): add GameSystem enum and Showcase DTOs

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

src/GmRelay.Shared/Features/Showcase/ShowcaseFilter.cs

@@ -0,0 +1,23 @@
+namespace GmRelay.Shared.Features.Showcase;
+
+public sealed record ShowcaseFilter(
+    DateFilter Date = DateFilter.All,
+    SeatFilter Seats = SeatFilter.Any,
+    string? System = null,
+    bool? IsOneShot = null,
+    string? Format = null);
+
+public enum DateFilter
+{
+    Today,
+    Tomorrow,
+    ThisWeek,
+    All
+}
+
+public enum SeatFilter
+{
+    Available,
+    Waitlist,
+    Any
+}

src/GmRelay.Shared/Features/Showcase/ShowcaseSessionDto.cs

@@ -0,0 +1,19 @@
+namespace GmRelay.Shared.Features.Showcase;
+
+public sealed record ShowcaseSessionDto(
+    Guid Id,
+    Guid GroupId,
+    string GroupName,
+    string? GroupSlug,
+    string Title,
+    DateTime ScheduledAt,
+    string Status,
+    string? System,
+    bool IsOneShot,
+    string? Format,
+    int? DurationMinutes,
+    string? CoverImageUrl,
+    int? MaxPlayers,
+    int ActivePlayerCount,
+    int WaitlistedPlayerCount,
+    bool AllowDirectRegistration);