change answer logic to support multiple answer questions

SurveyLib.Core/Models/Answer.cs

@@ -2,6 +2,8 @@ namespace SurveyLib.Core.Models;
 
 public class Answer
 {
+    public int Id { get; set; }
+
     public int CompletionId { get; set; }
     public int QuestionId { get; set; }
     public string AnswerText { get; set; }

SurveyLib.Infrastructure.EFCore/Data/SurveyDbContext.cs

@@ -24,6 +24,6 @@ public class SurveyDbContext : DbContext
 
     protected override void OnModelCreating(ModelBuilder modelBuilder)
     {
-        modelBuilder.Entity<Answer>().HasKey(a => new { a.CompletionId, a.QuestionId });
+        modelBuilder.Entity<Answer>().HasIndex(a => new { a.CompletionId, a.QuestionId, a.AnswerText }).IsUnique();
     }
 }