feat: update callback handler and repository for user

backend/internal/storage/models.go

@@ -25,3 +25,10 @@ type User struct {
 	GithubLogin string `db:"github_login"`
 	AvatarURL   string `db:"avatar_url"`
 }
+
+type UserReg struct {
+	Email       string `db:"email"`
+	GithubID    string `db:"github_id"`
+	GithubLogin string `db:"github_login"`
+	AvatarURL   string `db:"avatar_url"`
+}