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]>

tests/Unit/Offering/OfferingTest.php

@@ -277,4 +277,48 @@ class OfferingTest extends TestCase
         self::assertContains(Offering::BILLING_ONE_TIME, Offering::VALID_BILLING_MODES);
         self::assertContains(Offering::BILLING_FULL_TERM, Offering::VALID_BILLING_MODES);
     }
+
+    public function testEffectiveEnrollmentDeadlineDefaultsToTermStart(): void
+    {
+        $offering = new Offering(1, Offering::KIND_GROUP_CLASS, 'Choir', termStart: '2026-09-08');
+
+        self::assertSame('2026-09-08', $offering->effectiveEnrollmentDeadline());
+    }
+
+    public function testEffectiveEnrollmentDeadlineUsesExplicitValueWhenSet(): void
+    {
+        $offering = new Offering(1, Offering::KIND_GROUP_CLASS, 'Choir', termStart: '2026-09-08', enrollmentDeadline: '2026-08-31');
+
+        self::assertSame('2026-08-31', $offering->effectiveEnrollmentDeadline());
+    }
+
+    public function testEffectiveEnrollmentDeadlineIsNullWithoutDates(): void
+    {
+        $offering = new Offering(1, Offering::KIND_GROUP_CLASS, 'Choir');
+
+        self::assertNull($offering->effectiveEnrollmentDeadline());
+    }
+
+    public function testIsEnrollmentOpenOnAndBeforeTheDeadlineDay(): void
+    {
+        $offering = new Offering(1, Offering::KIND_GROUP_CLASS, 'Choir', termStart: '2026-09-08', enrollmentDeadline: '2026-08-31');
+
+        self::assertTrue($offering->isEnrollmentOpen('2026-08-30'));
+        self::assertTrue($offering->isEnrollmentOpen('2026-08-31'));
+        self::assertFalse($offering->isEnrollmentOpen('2026-09-01'));
+    }
+
+    public function testIsEnrollmentOpenAlwaysTrueWithoutADeadline(): void
+    {
+        $offering = new Offering(1, Offering::KIND_GROUP_CLASS, 'Choir');
+
+        self::assertTrue($offering->isEnrollmentOpen('2099-01-01'));
+    }
+
+    public function testToArrayIncludesEnrollmentDeadline(): void
+    {
+        $offering = new Offering(1, Offering::KIND_GROUP_CLASS, 'Choir', enrollmentDeadline: '2026-08-31', id: 10);
+
+        self::assertSame('2026-08-31', $offering->toArray()['enrollment_deadline']);
+    }
 }