Harden booking, offering exposure, payments, and invites

Security fixes from a pen-test review (issues #31–#37):

- #31 Booking no longer trusts a client-supplied offering_id: a slot-tied
  offering is authoritative and any offering used must belong to the slot's
  instructor, closing a free/misrouted-payment bypass.
- #34 Availability slot creation rejects an offering the instructor does not
  own (AvailabilityEndpoint now takes OfferingRepository).
- #32 Offering/question/policy listing endpoints now require book_lesson
  instead of being public (no anonymous consumer exists); Offering::toArray
  also omits etransfer_email from listings as defense-in-depth.
- #33 Slots are claimed atomically (UPDATE ... WHERE is_booked = 0) before a
  lesson is inserted, preventing a double-booking race.
- #35 A single weekly booking is capped (MAX_WEEKLY_OCCURRENCES) and only
  creates lessons for slots it actually claimed.
- #36 Stripe secret/webhook keys are write-only in the settings UI and a blank
  submit keeps the stored value; secrets are never echoed back into HTML.
- #37 Pending invites expire after 14 days (Invite::isAcceptable), enforced at
  registration and surfaced on the admin invites list.

Adds BookingEndpointTest plus Invite/Offering/AvailabilityRepository coverage
and minimal WP_REST_Request/WP_REST_Response stubs. composer test (200),
lint, and cs all green.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/Unit/Offering/OfferingTest.php

@@ -84,6 +84,21 @@ class OfferingTest extends TestCase
         }
     }
 
+    public function testToArrayIncludesEtransferEmailByDefault(): void
+    {
+        $offering = new Offering(1, Offering::KIND_PRIVATE_LESSON, 'Lesson', etransferEmail: 'studio@example.com', id: 10);
+
+        self::assertArrayHasKey('etransfer_email', $offering->toArray());
+        self::assertSame('studio@example.com', $offering->toArray()['etransfer_email']);
+    }
+
+    public function testToArrayOmitsEtransferEmailWhenExcluded(): void
+    {
+        $offering = new Offering(1, Offering::KIND_PRIVATE_LESSON, 'Lesson', etransferEmail: 'studio@example.com', id: 10);
+
+        self::assertArrayNotHasKey('etransfer_email', $offering->toArray(includeEtransferEmail: false));
+    }
+
     public function testValidKindAndBillingConstants(): void
     {
         self::assertContains(Offering::KIND_PRIVATE_LESSON, Offering::VALID_KINDS);