Add group-class withdrawal deadline and kind-aware offering form

Group classes now carry an optional per-class withdrawal deadline. Up to
that day a student may withdraw themselves from the class; the withdrawal
frees the seat and voids any pending payment but never issues an account
credit. After the deadline self-withdrawal closes and a studio admin must
withdraw the student by hand (the admin path is never subject to the
deadline). A blank deadline keeps self-withdrawal open indefinitely.

Also make the Add/Edit Offering form show only the fields relevant to the
selected kind: group settings for group classes, weekly reservation for
private lessons. Progressive enhancement — without JS every field renders.

- New nullable us_offerings.withdrawal_deadline column; Offering model gains
  $withdrawalDeadline + isWithdrawalOpen().
- New student endpoint POST /enrollments/{id}/withdraw, gated by the deadline
  (403 withdrawal_closed), ownership-checked, idempotent.
- Front-end group-class page shows a Withdraw button while open.
- No USC_VERSION bump: 1.2.0 is unreleased and accumulates schema changes
  under its section, matching the scheduled-billing and credit features.

Tests: composer test (596), composer lint, composer cs all pass.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

CHANGELOG.md

@@ -17,6 +17,8 @@ each change under the current top section as you work.
 - Offerings can now bill on a schedule: **weekly** (a pending payment 24 hours before each lesson) or **monthly** (one payment on the 1st for that month's lessons), alongside the existing one-time and full-term modes. Applies to both private lessons and group classes. A daily job generates due payments, and each student receives one consolidated itemised email per scan; batched payments share a reference so the admin Payments queue groups them with a lump-sum total for e-transfer reconciliation. Cancelling a lesson never voids a scheduled payment.
 - Cancelling a lesson that was **already paid for** now credits the student that money instead of leaving it as a manual refund. The credit is one lesson's share of what they paid — the whole amount for a single lesson, or a per-lesson slice of a monthly charge or a full-term series. The daily billing scan automatically applies any available credit against a student's upcoming weekly/monthly charges before emailing their notice, which shows the credit applied and the reduced total due; a charge fully covered by credit is settled and leaves the admin Payments queue. A student's outstanding credit balance is shown on their **student detail** page in the studio admin. Still-pending (unpaid) payments continue to be voided on cancellation as before.
 - 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. While enrolment is open, each class card shows an "Enrol by" date.
+- Group classes now also carry a **withdrawal deadline** the instructor sets per class. Up to that day a student can withdraw themselves from the class (the group-class page shows a **Withdraw** button) — this frees their seat and voids any pending payment but does **not** credit their account. After the deadline self-withdrawal closes and the student must ask the studio, who can still withdraw them by hand from the student detail page. Leaving the deadline blank keeps self-withdrawal open indefinitely.
+- The **Add/Edit Offering** form now shows only the fields relevant to the selected kind: the group-class settings (capacity, dates, times, enrolment/withdrawal deadlines, sessions, schedule note, invite-only) appear only for a group class, and the weekly-reservation option only for a private lesson.
 - 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.
 - Studio admins and instructors can open a **lesson detail view** from the Scheduler and My Lessons lists, showing the offering booked, the policy versions the student accepted (with acceptance time and IP), and their intake answers. On My Lessons an instructor may only open their own lessons; the studio Scheduler may open any.