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/Offering/OfferingController.php

@@ -209,6 +209,10 @@ class OfferingController {
 
 		$classTime = Offering::normalizeTime( sanitize_text_field( Val::string( wp_unslash( $_POST['class_time'] ?? '' ) ) ) );
 
+		// A blank (or invalid) deadline means "use the default" — the first class
+		// day (term_start), applied by Offering::effectiveEnrollmentDeadline().
+		$enrollmentDeadline = Offering::normalizeDate( sanitize_text_field( Val::string( wp_unslash( $_POST['enrollment_deadline'] ?? '' ) ) ) );
+
 		return new Offering(
 			instructorId:    $this->resolveInstructorId( $instructorId, $manageAll, $existing ),
 			kind:            $kind,
@@ -223,6 +227,7 @@ class OfferingController {
 			termStart:       $termStart,
 			termEnd:         $termEnd,
 			classTime:       $classTime,
+			enrollmentDeadline: $enrollmentDeadline,
 			scheduleNote:    $this->nullableText( sanitize_text_field( Val::string( wp_unslash( $_POST['schedule_note'] ?? '' ) ) ) ),
 			etransferEmail:  $this->nullableText( sanitize_email( Val::string( wp_unslash( $_POST['etransfer_email'] ?? '' ) ) ) ),
 			cancellationCutoffHours: $cutoffHours,