feat: normalize subscription architecture, integrate pgvector, and implement Stripe webhook subscription management.

src/NexusReader.UI.Shared/Services/KnowledgeCoordinator.cs

@@ -98,6 +98,12 @@ public sealed class KnowledgeCoordinator : IDisposable
         return null;
     }
 
+    public void Clear()
+    {
+        _graphService.Clear();
+        _quizService.SetQuiz(null, null);
+    }
+
     public void Dispose()
     {
         _interactionService.OnNodeSelected -= HandleNodeSelected;