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/Availability/AvailabilityRepositoryTest.php

@@ -116,16 +116,25 @@ class AvailabilityRepositoryTest extends TestCase
         self::assertSame(5, $slot->instructorId);
     }
 
-    public function testMarkBookedUpdatesRecord(): void
+    public function testClaimReturnsTrueWhenSlotWasUnbooked(): void
     {
         $this->db->shouldReceive('update')
             ->once()
-            ->with('wp_us_availability', ['is_booked' => 1], ['id' => 7], ['%d'], ['%d'])
+            ->with('wp_us_availability', ['is_booked' => 1], ['id' => 7, 'is_booked' => 0], ['%d'], ['%d', '%d'])
             ->andReturn(1);
 
-        $result = $this->repo->markBooked(7);
+        self::assertTrue($this->repo->claim(7));
+    }
 
-        self::assertTrue($result);
+    public function testClaimReturnsFalseWhenSlotAlreadyBooked(): void
+    {
+        // The is_booked = 0 guard matches no row once the slot is taken.
+        $this->db->shouldReceive('update')
+            ->once()
+            ->with('wp_us_availability', ['is_booked' => 1], ['id' => 7, 'is_booked' => 0], ['%d'], ['%d', '%d'])
+            ->andReturn(0);
+
+        self::assertFalse($this->repo->claim(7));
     }
 
     public function testDeleteReturnsFalseWhenRowNotDeleted(): void