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/Common/ShowactApplicationSchedule.cs

@@ -0,0 +1,36 @@
+using Backend.Domain;
+
+namespace Backend.Common;
+
+public static class ShowactApplicationSchedule
+{
+    public static string? Validate(DateOnly? startsAt, DateOnly? endsAt)
+    {
+        if (startsAt.HasValue && endsAt.HasValue && startsAt.Value > endsAt.Value)
+        {
+            return "Der Showact-Zeitraum ist ungueltig. Der Start darf nicht nach der Deadline liegen.";
+        }
+
+        return null;
+    }
+
+    public static bool IsOpenNow(SiteSettings settings, DateOnly today)
+    {
+        if (!settings.ShowactApplicationsEnabled)
+        {
+            return false;
+        }
+
+        if (settings.ShowactApplicationStartsAt.HasValue && today < settings.ShowactApplicationStartsAt.Value)
+        {
+            return false;
+        }
+
+        if (settings.ShowactApplicationEndsAt.HasValue && today > settings.ShowactApplicationEndsAt.Value)
+        {
+            return false;
+        }
+
+        return true;
+    }
+}