Add Offerings domain and studio-admin capabilities

Implements the offerings catalog (#1): private-lesson types and group
classes carrying pricing, billing mode (one_time/full_term), duration,
capacity, and term details. Adds the src/Offering/ domain (value object,
repository, REST endpoint, admin controller + template), the us_offerings
table, and an Offerings admin page.

Also lands the capability slice of #9: registers the us_studio_admin role
and the new capability strings (manage_instructors, manage_offerings,
manage_questions, manage_policies, manage_billing, view_all_payments,
view_own_payments, export_payments) so offering management gates correctly.

Tests: tests/Unit/Offering/ (value object + repository) and a studio-admin
case in RoleManagerTest. composer test, cs, and PHPStan level 6 all pass.

Refs #1 #9

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

src/Plugin.php

@@ -6,6 +6,7 @@ namespace Unsupervised\Schedular;
 use Unsupervised\Schedular\Auth\RoleManager;
 use Unsupervised\Schedular\Availability\AvailabilityRepository;
 use Unsupervised\Schedular\Booking\BookingRepository;
+use Unsupervised\Schedular\Offering\OfferingRepository;
 
 class Plugin {
 
@@ -15,10 +16,11 @@ class Plugin {
 		global $wpdb;
 		$availability = new AvailabilityRepository( $wpdb );
 		$bookings     = new BookingRepository( $wpdb );
+		$offerings    = new OfferingRepository( $wpdb );
 
 		( new RoleManager() )->register();
-		( new AdminMenu( $availability, $bookings ) )->register();
-		( new RestRegistrar( $availability, $bookings ) )->register();
+		( new AdminMenu( $availability, $bookings, $offerings ) )->register();
+		( new RestRegistrar( $availability, $bookings, $offerings ) )->register();
 		( new ShortcodeRegistrar() )->register();
 	}
 }