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

src/NexusReader.Domain/Entities/SubscriptionPlan.cs

@@ -0,0 +1,30 @@
+using System.ComponentModel.DataAnnotations;
+
+namespace NexusReader.Domain.Entities;
+
+public class SubscriptionPlan
+{
+    public const string FreeName = "Free";
+    public const string BasicName = "Basic";
+    public const string ProName = "Pro";
+    public const string EnterpriseName = "Enterprise";
+
+    public const int FreeId = 1;
+    public const int BasicId = 2;
+    public const int ProId = 3;
+    public const int EnterpriseId = 4;
+
+    [Key]
+    public int Id { get; set; }
+
+    [Required]
+    [MaxLength(50)]
+    public string PlanName { get; set; } = string.Empty;
+
+    public int AITokenLimit { get; set; }
+
+    public decimal MonthlyPrice { get; set; }
+
+    [MaxLength(50)]
+    public string StripeProductId { get; set; } = string.Empty;
+}