The booking mode locks after the first save because existing appointments were
computed under that mode's rules. A lock with no key, though, traps a practice
that picked the wrong mode on day one, so ROLE_ADMIN can now open it.
The first attempt is still refused when the doctor has active appointments in
the next year, and says how many; the admin repeats the request with
force_mode_change to confirm they know what happens to those. The flag does
nothing for anyone else. GET now returns booking_mode_changeable so the panel
enables the toggle from the server's answer rather than guessing from the role.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
