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]>

tests/Unit/Offering/OfferingTest.php

@@ -84,6 +84,7 @@ class OfferingTest extends TestCase
             'term_end'         => '2027-06-30',
             'schedule_note'    => 'Tuesdays 4:00pm',
             'etransfer_email'  => null,
+            'cancellation_cutoff_hours' => '48',
             'is_active'        => '1',
         ];
 
@@ -95,9 +96,37 @@ class OfferingTest extends TestCase
         self::assertSame(120.00, $offering->price);
         self::assertSame(20, $offering->capacity);
         self::assertSame(Offering::BILLING_FULL_TERM, $offering->billingMode);
+        self::assertSame(48, $offering->cancellationCutoffHours);
         self::assertTrue($offering->isActive);
     }
 
+    public function testFromRowMapsNullCancellationCutoff(): void
+    {
+        $row = (object) [
+            'id'               => '7',
+            'instructor_id'    => '3',
+            'kind'             => Offering::KIND_PRIVATE_LESSON,
+            'title'            => '30 min lesson',
+            'description'      => null,
+            'duration_minutes' => '30',
+            'price'            => '40.00',
+            'currency'         => 'CAD',
+            'billing_mode'     => Offering::BILLING_ONE_TIME,
+            'allow_weekly'     => '0',
+            'capacity'         => null,
+            'term_start'       => null,
+            'term_end'         => null,
+            'schedule_note'    => null,
+            'etransfer_email'  => null,
+            'cancellation_cutoff_hours' => null,
+            'is_active'        => '1',
+        ];
+
+        $offering = Offering::fromRow($row);
+
+        self::assertNull($offering->cancellationCutoffHours);
+    }
+
     public function testToArrayContainsExpectedKeys(): void
     {
         $offering = new Offering(1, Offering::KIND_PRIVATE_LESSON, 'Lesson', id: 10);