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

src/NexusReader.Domain/Entities/SemanticKnowledgeCache.cs

@@ -1,5 +1,6 @@
 using System.ComponentModel.DataAnnotations;
 using System.ComponentModel.DataAnnotations.Schema;
+using Pgvector;
 
 namespace NexusReader.Domain.Entities;
 
@@ -27,8 +28,7 @@ public class SemanticKnowledgeCache
     [MaxLength(128)]
     public string TenantId { get; set; } = string.Empty;
 
-    [Column(TypeName = "vector(1536)")] // text-embedding-004 has 768 or 1536 dims, assuming 1536 for high-fidelity
-    public float[]? Vector { get; set; }
+    public Vector? Vector { get; set; }
 
     public DateTime CreatedAt { get; set; } = DateTime.UtcNow;
 }