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]>

templates/admin/offerings.php

@@ -112,6 +112,13 @@ if ($editing && null !== $editing->termStart && null !== $editing->termEnd && $e
                     <span class="description"><?php esc_html_e('Group classes only — the time each session starts. Combined with the duration to block the instructor’s availability.', 'unsupervised-schedular'); ?></span>
                 </td>
             </tr>
+            <tr>
+                <th><label for="enrollment_deadline"><?php esc_html_e('Enrolment deadline', 'unsupervised-schedular'); ?></label></th>
+                <td>
+                    <input type="date" name="enrollment_deadline" id="enrollment_deadline" value="<?php echo esc_attr($editing->enrollmentDeadline ?? ''); ?>">
+                    <p class="description"><?php esc_html_e('Group classes only — the last day students may enrol. Leave blank to default to the first day of the class.', 'unsupervised-schedular'); ?></p>
+                </td>
+            </tr>
             <tr>
                 <th><?php esc_html_e('Sessions', 'unsupervised-schedular'); ?></th>
                 <td>