Grant studio-admin capabilities to WordPress administrators

WordPress administrators (manage_options) now implicitly hold every
studio-admin capability via a user_has_cap filter, so the site owner runs
the studio without being assigned the separate us_studio_admin role. The
grant persists nothing and is removed on deactivation. The us_studio_admin
role still exists for non-administrator staff and does NOT confer any core
WordPress admin powers.

Also re-gate the studio-wide "Scheduler" dashboard off manage_options onto
a new view_all_lessons capability (added to the studio-admin cap set), so a
us_studio_admin user can see it too — previously it was administrator-only.

- RoleManager: STUDIO_ADMIN_CAPS constant, CAP_VIEW_ALL_LESSONS,
  grantStudioCapsToAdministrators() user_has_cap filter
- AdminMenu + LessonController: Scheduler gated on view_all_lessons
- Docs: user-roles.md cap matrix + administrator note; lesson-booking.md
- Tests: administrators receive studio caps; non-admins do not

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/features/lesson-booking.md

@@ -53,7 +53,7 @@ Group classes follow the same registration flow but enrol against an offering of
 kind `group_class`; see `group-classes.md`.
 
 ## Admin Interface
-- **Scheduler** (`manage_options` only): all upcoming lessons across all instructors
+- **Scheduler** (`view_all_lessons` — studio admin / administrators): all upcoming lessons across all instructors
 - **My Lessons** (`view_own_lessons`): upcoming lessons for the logged-in instructor
 
 ## Frontend Shortcodes