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/Schema.php

@@ -64,6 +64,7 @@ class Schema {
                 term_start       DATE            DEFAULT NULL,
                 term_end         DATE            DEFAULT NULL,
                 class_time       TIME            DEFAULT NULL,
+                enrollment_deadline DATE         DEFAULT NULL,
                 schedule_note    VARCHAR(191)    DEFAULT NULL,
                 etransfer_email  VARCHAR(191)    DEFAULT NULL,
                 cancellation_cutoff_hours SMALLINT UNSIGNED DEFAULT NULL,