Add lesson booking registration flow (offering, questions, policies)

Implements #3: students register for a private lesson by picking a slot,
answering the offering's intake questions, and accepting booking-scoped
policies. Payment is a clean seam for #7 (lessons land pending; payment_id
null; instructor confirms via PATCH /bookings/{id}/status).

- Schema: us_lessons += offering_id, recurrence, series_id, payment_id.
- Lesson: new fields + recurrence constants.
- BookingRepository::insertSeries() builds a weekly series sharing a
  series_id; AvailabilityRepository::findUnbookedInGroup() reserves a group.
- RegistrationGate (src/Registration/): validate + record intake answers and
  booking-scoped policy acceptances. Reused by group enrolment (#4).
- BookingEndpoint::book(): offering_id, recurrence, answers,
  accepted_policy_version_ids; single or weekly; records answers/acceptances
  (type lesson).
- GET /policies?scope=booking filter.
- Front-end booking.js: slot -> questions + policies -> submit.
- Wiring: RegistrationGate built in Plugin, passed via RestRegistrar.
- Test-only WP_Error stub in tests/bootstrap.php for gate testing.

Refs #3

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

tests/Unit/Booking/LessonTest.php

@@ -28,41 +28,52 @@ class LessonTest extends TestCase
         $lesson = new Lesson(slotId: 1, studentId: 2, instructorId: 3);
 
         self::assertSame(Lesson::STATUS_PENDING, $lesson->status);
+        self::assertNull($lesson->offeringId);
+        self::assertSame(Lesson::RECURRENCE_SINGLE, $lesson->recurrence);
+        self::assertNull($lesson->seriesId);
+        self::assertNull($lesson->paymentId);
         self::assertNull($lesson->notes);
         self::assertNull($lesson->id);
     }
 
+    public function testRecurrenceConstants(): void
+    {
+        self::assertContains(Lesson::RECURRENCE_SINGLE, Lesson::VALID_RECURRENCES);
+        self::assertContains(Lesson::RECURRENCE_WEEKLY, Lesson::VALID_RECURRENCES);
+    }
+
     public function testFromRowMapsCorrectly(): void
     {
         $row = (object) [
             'id'            => '99',
             'slot_id'       => '10',
+            'offering_id'   => '7',
             'student_id'    => '20',
             'instructor_id' => '30',
+            'recurrence'    => Lesson::RECURRENCE_WEEKLY,
+            'series_id'     => '99',
             'status'        => 'confirmed',
+            'payment_id'    => null,
             'notes'         => 'Bring your guitar.',
         ];
 
         $lesson = Lesson::fromRow($row);
 
         self::assertSame(99, $lesson->id);
-        self::assertSame(10, $lesson->slotId);
-        self::assertSame(20, $lesson->studentId);
-        self::assertSame(30, $lesson->instructorId);
+        self::assertSame(7, $lesson->offeringId);
+        self::assertSame(Lesson::RECURRENCE_WEEKLY, $lesson->recurrence);
+        self::assertSame(99, $lesson->seriesId);
+        self::assertNull($lesson->paymentId);
         self::assertSame('confirmed', $lesson->status);
-        self::assertSame('Bring your guitar.', $lesson->notes);
     }
 
     public function testToArrayContainsExpectedKeys(): void
     {
-        $lesson = new Lesson(1, 2, 3, Lesson::STATUS_PENDING, 'Note', 5);
+        $lesson = new Lesson(slotId: 1, studentId: 2, instructorId: 3, notes: 'Note', id: 5);
         $arr    = $lesson->toArray();
 
-        self::assertArrayHasKey('id', $arr);
-        self::assertArrayHasKey('slot_id', $arr);
-        self::assertArrayHasKey('student_id', $arr);
-        self::assertArrayHasKey('instructor_id', $arr);
-        self::assertArrayHasKey('status', $arr);
-        self::assertArrayHasKey('notes', $arr);
+        foreach (['id', 'slot_id', 'offering_id', 'student_id', 'instructor_id', 'recurrence', 'series_id', 'status', 'payment_id', 'notes'] as $key) {
+            self::assertArrayHasKey($key, $arr);
+        }
     }
 }