Add cancellation cutoff limiting how close to a lesson a student can cancel

Students can no longer cancel their own lesson online once it starts within a
configured window; instructors and studio admins can always cancel.

- Studio default `us_cancellation_cutoff_hours` (stored/computed in hours,
  entered and displayed in days under Studio Settings → Cancellations).
- Optional per-offering override `cancellation_cutoff_hours` (entered in hours);
  blank inherits the studio default, 0 allows anytime cancellation.
- `Booking\CancellationPolicy` resolves the effective window and decides;
  `BookingEndpoint::cancel()` returns a 403 `cancellation_closed` when too late.
  The instructor status endpoint and studio-admin student actions bypass it.

Closes #93

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

docs/features/cancellation-cutoff.md

@@ -0,0 +1,69 @@
+# Feature: Cancellation Cutoff
+
+## Overview
+Students may cancel their own lessons online — but not indefinitely close to the
+start time. A **cancellation cutoff** closes student-initiated cancellation once
+a lesson begins within a configured window. Instructors and studio admins are
+never subject to the cutoff: they can cancel a lesson at any time through the
+lesson-status and student-management flows.
+
+The window is resolved per lesson:
+
+1. If the lesson's **offering** sets its own cutoff, that value is used.
+2. Otherwise the **studio default** applies.
+
+Both values are expressed and computed in **hours**. The studio default is
+entered and displayed to the admin in **days** for convenience; a per-offering
+override is entered directly in hours (a finer-grained "time").
+
+## Data Model
+
+### Option `us_cancellation_cutoff_hours`
+Studio-wide default cutoff, stored as an integer number of **hours**. Defaults to
+`24` (one day) when unset. `0` means students may cancel at any time.
+
+### Column `{prefix}us_offerings.cancellation_cutoff_hours`
+Nullable `SMALLINT UNSIGNED`. `NULL` means "inherit the studio default"; any set
+value (including `0` — cancel any time) overrides it for that offering.
+
+## Resolution & Enforcement
+`Booking\CancellationPolicy` owns the logic:
+
+- `cutoffHours(?int $offeringCutoffHours): int` — the offering's override when it
+  is a non-negative value, otherwise the studio default.
+- `studentMayCancel(string $slotStartDt, ?int $offeringCutoffHours, ?string $now): bool`
+  — false once `now` is within the effective cutoff of the slot start. A zero
+  cutoff always allows cancellation; unparseable datetimes fail open so a student
+  is never trapped by bad data. Comparisons use WordPress-local time
+  (`current_time('mysql')`), matching how upcoming lessons are computed.
+- `describeCutoff(int $hours): string` — humanises a cutoff for messages
+  (whole days as days, otherwise hours).
+
+`Booking\BookingEndpoint::cancel()` (the student endpoint,
+`POST /bookings/{id}/cancel`) consults the policy before cancelling and returns a
+`cancellation_closed` (HTTP 403) error explaining the window when it is too late.
+The instructor status endpoint (`PATCH /bookings/{id}/status`) and
+`Auth\StudentActions::cancelLesson()` (studio-admin student view) bypass the
+policy entirely.
+
+## Admin Interface
+- **Studio Settings → Cancellations**: "Cancellation cutoff (days)" — the studio
+  default, entered/displayed in days, stored in hours.
+- **Offerings** add/edit form: "Cancellation cutoff (hours)" — an optional
+  per-offering override; blank inherits the studio default, `0` allows anytime
+  cancellation.
+
+## Implementation
+- Service: `Unsupervised\Schedular\Booking\CancellationPolicy`
+- Studio default: `Unsupervised\Schedular\Payment\StudioSettings::cancellationCutoffHours()`
+  (option `us_cancellation_cutoff_hours`)
+- Per-offering value: `Unsupervised\Schedular\Offering\Offering::$cancellationCutoffHours`
+- Enforcement: `Unsupervised\Schedular\Booking\BookingEndpoint::cancel()`
+- Wiring: `RestRegistrar` constructs `new CancellationPolicy( new StudioSettings() )`
+
+## Tests
+- `tests/Unit/Booking/CancellationPolicyTest.php`
+- `tests/Unit/Booking/BookingEndpointTest.php` (cutoff cases in `cancel()`)
+- `tests/Unit/Payment/StudioSettingsTest.php` (default getter)
+- `tests/Unit/Offering/OfferingTest.php`, `tests/Unit/Offering/OfferingRepositoryTest.php`
+  (new column round-trips)

docs/features/offerings.md

@@ -21,6 +21,7 @@ An offering is anything a student can register for: a private-lesson type (30 or
 | `term_start`       | DATE             | Group / term offerings — first day; NULL otherwise          |
 | `term_end`         | DATE             | Group / term offerings — last day; NULL otherwise           |
 | `schedule_note`    | VARCHAR(191)     | Group only — human-readable schedule, e.g. "Tuesdays 4:00pm"|
+| `cancellation_cutoff_hours` | SMALLINT UNSIGNED | Optional per-offering cancellation cutoff in hours; NULL inherits the studio default (see `cancellation-cutoff.md`) |
 | `is_active`        | TINYINT(1)       | 0 = hidden from registration, 1 = bookable                  |
 | `created_at`       | DATETIME         | Insertion time                                              |