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>

src/Policy/PolicyEndpoint.php

@@ -21,7 +21,7 @@ class PolicyEndpoint {
 				[
 					'methods'             => \WP_REST_Server::READABLE,
 					'callback'            => [ $this, 'index' ],
-					'permission_callback' => '__return_true',
+					'permission_callback' => [ $this, 'canBook' ],
 				],
 				[
 					'methods'             => \WP_REST_Server::CREATABLE,
@@ -185,6 +185,16 @@ class PolicyEndpoint {
 		return is_user_logged_in() && current_user_can( RoleManager::CAP_MANAGE_POLICIES );
 	}
 
+	/**
+	 * The published-policy listing is only read by the logged-in student
+	 * booking/enrolment flow (the signup gate renders its policies server-side, not
+	 * via this endpoint), so reading it requires the booking capability — there is
+	 * no anonymous consumer.
+	 */
+	public function canBook(): bool {
+		return is_user_logged_in() && current_user_can( RoleManager::CAP_BOOK_LESSON );
+	}
+
 	/**
 	 * Load the version named in the route and confirm it belongs to the policy.
 	 */