Add group-class enrolment deadline with instructor late-enrolment override

Group classes gain an instructor-set enrolment deadline (new
us_offerings.enrollment_deadline column) that defaults to the first day of
the class (term_start). Past the deadline students can no longer self-enrol:
the enrolment endpoint rejects it (403 enrollment_closed) and the front-end
class list shows "Enrolment has closed." in place of the Enrol button.

Instructors keep a manual path: the "Add students directly" control on each
class's details page now renders for public classes too (not just
invite-only) and deliberately bypasses the deadline and capacity, so a
student can be added as a late enrolment after the class has closed. Past
the deadline the details page labels these as late enrolments.

Bumps USC_VERSION to 1.1.3 for the schema change.

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

src/GroupClass/EnrollmentEndpoint.php

@@ -95,6 +95,12 @@ class EnrollmentEndpoint {
 			return new \WP_Error( 'invite_required', __( 'This class is by invitation only.', 'unsupervised-schedular' ), [ 'status' => 403 ] );
 		}
 
+		// Enrolment closes at the end of the deadline day — the instructor's set
+		// deadline, or the first class day by default.
+		if ( ! $offering->isEnrollmentOpen( Val::string( current_time( 'Y-m-d' ) ) ) ) {
+			return new \WP_Error( 'enrollment_closed', __( 'Enrolment for this class has closed.', 'unsupervised-schedular' ), [ 'status' => 403 ] );
+		}
+
 		if ( null !== $offering->capacity && $this->enrollments->countActiveForOffering( $offeringId ) >= $offering->capacity ) {
 			return new \WP_Error( 'class_full', __( 'This class is full.', 'unsupervised-schedular' ), [ 'status' => 409 ] );
 		}