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

src/NexusReader.Domain/Entities/KnowledgeUnit.cs

@@ -1,6 +1,7 @@
 using System.ComponentModel.DataAnnotations;
 using System.ComponentModel.DataAnnotations.Schema;
 using NexusReader.Domain.Enums;
+using Pgvector;
 
 namespace NexusReader.Domain.Entities;
 
@@ -30,8 +31,7 @@ public class KnowledgeUnit
     [MaxLength(128)]
     public string TenantId { get; set; } = string.Empty;
 
-    [Column(TypeName = "vector(768)")] // Default for text-embedding-004
-    public float[]? Vector { get; set; }
+    public Vector? Vector { get; set; }
 
     public DateTime CreatedAt { get; set; } = DateTime.UtcNow;