Add open student registration with email confirmation and approval

Students could previously join by invite only. Add an optional
self-approval mode, toggled from Studio Settings → Registration: anyone
may sign up on the existing [us_student_register] page, confirm their
email via a tokenised link, and then be approved by a studio admin
before the account is usable.

- Enabling the toggle mirrors WordPress's own membership settings
  (users_can_register + default_role = us_student) and snapshots their
  previous values so disabling restores them.
- WordPress's native registration form is blocked while open
  registration is on (login_init redirect + registration_errors
  fail-safe + register_url) so it cannot bypass signup policy acceptance.
- Pending accounts: unconfirmed email cannot log in; confirmed but
  unapproved can log in but the booking capability is withheld and the
  booking page shows an "awaiting approval" screen.
- Approve/reject from Students → Pending Students; reject hard-deletes
  the account so the email is freed to re-apply.
- Invite registration is unchanged; both modes coexist.

Account lifecycle lives in user meta (RegistrationStatus); no new tables.

Closes #63

Co-Authored-By: Claude Opus 4.8 <[email protected]>

README.md

@@ -31,7 +31,7 @@ model, REST API, classes, and tests. For contributor/architecture guidance see
 | Availability (durations, weekly recurrence, calendar) | [availability-management.md](docs/features/availability-management.md) | ✅ Implemented |
 | Registration questions (per-offering intake) | [registration-questions.md](docs/features/registration-questions.md) | ✅ Implemented |
 | Policies (drafting, versioning, tracked acceptance) | [policies.md](docs/features/policies.md) | ✅ Implemented |
-| Account registration (invite-only, signup policy acceptance) | [account-registration.md](docs/features/account-registration.md) | ✅ Implemented |
+| Account registration (invite or open self-approval, email confirmation, signup policy acceptance) | [account-registration.md](docs/features/account-registration.md) | ✅ Implemented |
 | Lesson booking (offering → questions → policies) | [lesson-booking.md](docs/features/lesson-booking.md) | ✅ Implemented |
 | Group classes (capacity-enforced enrolment) | [group-classes.md](docs/features/group-classes.md) | ✅ Implemented |
 | Student administration (studio-admin view) | [student-administration.md](docs/features/student-administration.md) | ✅ Implemented |
@@ -49,7 +49,7 @@ model, REST API, classes, and tests. For contributor/architecture guidance see
 | `[us_booking]` | Student calendar + private-lesson registration flow |
 | `[us_group_classes]` | Browse and enrol in group classes |
 | `[us_student_login]` | Front-end student login |
-| `[us_student_register]` | Invite-based account registration (accepts signup policies) |
+| `[us_student_register]` | Account registration — invite-based, or open self-signup with email confirmation + admin approval (accepts signup policies) |
 
 ## REST API