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

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

@@ -0,0 +1,25 @@
+namespace NexusReader.Application.DTOs.User;
+
+public record UserProfileDto
+{
+    public string Email { get; init; } = string.Empty;
+    public int AITokensUsed { get; init; }
+    
+    /// <summary>
+    /// Relational data for the current subscription plan.
+    /// </summary>
+    public SubscriptionPlanDto Plan { get; init; } = new();
+
+    public int AverageQuizScore { get; init; }
+    
+    /// <summary>
+    /// Summary of the last read book.
+    /// </summary>
+    public LastReadBookDto? LastReadBook { get; init; }
+}
+
+public record LastReadBookDto
+{
+    public Guid Id { get; init; }
+    public string Title { get; init; } = string.Empty;
+}