Fix error in SQL where a string was expected bit a number was provided

The SQL had to have quotes '', but it didn't.
This caused some DNIs that started with 0 (`09950031`) to be casted into `9950031`

backend/src/model/register.rs

@@ -88,7 +88,7 @@ impl RegisterCreate {
         } else {
             let res = sqlx::query!(
                 "SELECT MAX(register_code) AS max FROM register
-                WHERE register_course_id = ?",
+                WHERE register_course_id = '?'",
                 course_id
             )
             .fetch_one(db)