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

src/NexusReader.Application/DTOs/User/SubscriptionPlanDto.cs

@@ -0,0 +1,9 @@
+namespace NexusReader.Application.DTOs.User;
+
+public record SubscriptionPlanDto
+{
+    public int Id { get; init; }
+    public string Name { get; init; } = string.Empty;
+    public int AITokenLimit { get; init; }
+    public decimal MonthlyPrice { get; init; }
+}