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]>

CHANGELOG.md

@@ -11,6 +11,12 @@ When a `v*` tag is pushed, `.gitea/workflows/release.yml` publishes the matching
 the plugin to the next patch version and adds a fresh section here for it. Record
 each change under the current top section as you work.
 
+## [1.1.3]
+
+### Added
+- Group classes now carry an **enrolment deadline** the instructor sets on the offering. It defaults to the first day of the class, and once it passes students can no longer enrol — the enrolment page shows the class as closed and the API rejects late enrolments.
+- Instructors can add students to any group class by hand from its details page (**Add students directly**), which now appears for public classes too, not just invite-only ones. This bypasses the enrolment deadline and capacity, so a student can be enrolled as a **late enrolment** after the class has closed to self-enrolment.
+
 ## [1.1.2]
 
 ## [1.1.1]