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

src/NexusReader.Domain/Entities/Ebook.cs

@@ -23,6 +23,10 @@ public class Ebook
 
     public string? CoverUrl { get; set; }
 
+    [Required]
+    [MaxLength(128)]
+    public string TenantId { get; set; } = "global";
+
     public DateTime AddedDate { get; set; } = DateTime.UtcNow;
 
     public DateTime? LastReadDate { get; set; }