Prevent duplicate solve submissions for the same scramble

A competitor should not be able to time or submit the same scramble more than once unless explicitly editing their existing result.

Feedback source:
- Disallowing a solver to time the same scramble twice.
- Not allowing a solver to get access to a scramble they previously did on the timing area.
- Bug report: editing a past solve after a new scramble exists can hide the current scramble, as if the edit counted as the current solve.

Acceptance ideas:
- Treat each user/scramble pair as having at most one active result.
- Editing an old result must not mutate current scramble availability.
- Current scramble visibility should depend on the user's current attempt state, not edits to previous attempts.
- Add server-side validation where possible.

No se nombran archivos ni tests. Rastrea los puntos de entrada para el cronometraje, el envío, la visibilidad del scramble y la edición de resultados anteriores; luego inspecciona cómo se almacenan los resultados de usuario/scramble; se considera terminado cuando se rechazan los intentos duplicados, las ediciones preservan la visibilidad actual del scramble y la validación del lado del servidor está cubierta por tests.

Escrito por el modelo de indexación a partir del texto del issue.