Add viewer-range categories, nomination tracking, dynamic showact form, session timeout, share URLs, and workflow-per-season

Features:
- Category viewer ranges + subcategory templates (admin group modal, tree workspace)
- Nomination enrichment via TwitchTracker API (NominationEnrichmentService,
  TwitchTrackerViewerStatsProvider) with admin tracking rules editor
- Nomination group tracker: CategoryGroupName as primary identifier,
  CategoryId stays as nullable legacy field; StreamerIdentity table
- Dynamic showact application form builder (AdminShowactFormBuilder,
  ShowactApplicationSchedule)
- Session idle timeout setting (AdminSessionTimeoutCard)
- Share URLs for X and Discord (SiteSettings, public extras)
- Workflow rules now stored per season (falls back to global SiteSettings)
- New admin routes: settings/access, settings/workflows, tracking-rules
- New admin review workspace with subcategory tabs
- AdminCategoriesView rebuilt with group/subcategory modals

Migrations (all additive):
- AddShareUrls, AddShowactDynamicForm, AddCategoryViewerRanges,
  AddSessionIdleTimeoutSettings, AddSeasonSubcategoryTemplates,
  AddNominationGroupTrackerIdentity, AddShowactApplicationSchedule,
  AddSeasonWorkflowRulesJson

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

Backend/Extensions/ServiceCollectionExtensions.cs

@@ -26,6 +26,11 @@ public static class ServiceCollectionExtensions
         services.Configure<TwitchAuthOptions>(configuration.GetSection(TwitchAuthOptions.SectionName));
         services.AddMemoryCache();
         services.AddHttpClient();
+        services.AddHttpClient("TwitchTracker", client =>
+        {
+            client.Timeout = TimeSpan.FromSeconds(4);
+            client.DefaultRequestHeaders.UserAgent.ParseAdd("VTuberStarAwards/1.0");
+        });
         var allowedOrigins = ResolveAllowedOrigins(configuration, environment);
 
         var connectionString = configuration["VTSA_POSTGRES"] ?? configuration.GetConnectionString("Postgres");
@@ -88,6 +93,9 @@ public static class ServiceCollectionExtensions
         services.AddScoped<IRiskRuleService, RiskRuleService>();
         services.AddScoped<IRiskFlagService, RiskFlagService>();
         services.AddScoped<IAdminAuditService, AdminAuditService>();
+        services.AddScoped<IViewerStatsProvider, TwitchTrackerViewerStatsProvider>();
+        services.AddScoped<NominationTrackingReviewService>();
+        services.AddScoped<NominationEnrichmentService>();
         services.AddScoped<AdminSessionFilter>();
 
         return services;