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]>

src/Schema.php

@@ -65,6 +65,7 @@ class Schema {
                 term_end         DATE            DEFAULT NULL,
                 schedule_note    VARCHAR(191)    DEFAULT NULL,
                 etransfer_email  VARCHAR(191)    DEFAULT NULL,
+                cancellation_cutoff_hours SMALLINT UNSIGNED DEFAULT NULL,
                 is_active        TINYINT(1)      NOT NULL DEFAULT 1,
                 created_at       DATETIME        NOT NULL,
                 PRIMARY KEY (id),