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

src/NexusReader.Domain/Entities/QuizResult.cs

@@ -17,6 +17,10 @@ public class QuizResult
     [ForeignKey(nameof(UserId))]
     public NexusUser? User { get; set; }
 
+    [Required]
+    [MaxLength(128)]
+    public string TenantId { get; set; } = "global";
+
     [Required]
     public string Topic { get; set; } = string.Empty;