feat: Add RegisterAsync endpoint to AuthController (Refs #2)

Backend/PruebaGentle.Infrastructure/Repositories/UserRepository.cs

@@ -86,4 +86,21 @@ public class UserRepository : IUserRepository
             new { Username = username },
             commandType: CommandType.StoredProcedure);
     }
+
+    public async Task<User> RegisterAsync(User user)
+    {
+        using var connection = CreateConnection();
+        var result = await connection.QuerySingleAsync<User>(
+            "sp_User_Register",
+            new
+            {
+                user.Username,
+                user.PasswordHash,
+                user.Email,
+                user.NombreCompleto
+            },
+            commandType: CommandType.StoredProcedure);
+
+        return result;
+    }
 }