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/BookingRepositoryTest.php

@@ -34,19 +34,43 @@ class BookingRepositoryTest extends TestCase
                 Mockery::on(static function (array $data): bool {
                     return $data['slot_id'] === 10
                         && $data['student_id'] === 5
+                        && $data['offering_id'] === 7
+                        && $data['recurrence'] === Lesson::RECURRENCE_SINGLE
                         && $data['status'] === Lesson::STATUS_PENDING;
                 }),
-                ['%d', '%d', '%d', '%s', '%s', '%s']
+                ['%d', '%d', '%d', '%d', '%s', '%d', '%s', '%d', '%s', '%s']
             );
 
         $this->db->insert_id = 77;
 
-        $lesson = new Lesson(slotId: 10, studentId: 5, instructorId: 3);
+        $lesson = new Lesson(slotId: 10, studentId: 5, instructorId: 3, offeringId: 7);
         $result = $this->repo->insert($lesson);
 
         self::assertSame(77, $result);
     }
 
+    public function testInsertSeriesSharesSeriesIdAcrossSlots(): void
+    {
+        Functions\when('current_time')->justReturn('2026-04-01 12:00:00');
+
+        $ids = [40, 41, 42];
+        $this->db->shouldReceive('insert')
+            ->times(3)
+            ->andReturnUsing(function () use (&$ids): void {
+                $this->db->insert_id = array_shift($ids);
+            });
+
+        // The first lesson is back-filled with its own id as the series id.
+        $this->db->shouldReceive('update')
+            ->once()
+            ->with('wp_us_lessons', ['series_id' => 40], ['id' => 40], ['%d'], ['%d']);
+
+        $template = new Lesson(slotId: 0, studentId: 5, instructorId: 3, offeringId: 7);
+        $result   = $this->repo->insertSeries($template, [100, 101, 102]);
+
+        self::assertSame([40, 41, 42], $result);
+    }
+
     public function testFindByIdReturnsNullWhenNotFound(): void
     {
         $this->db->shouldReceive('prepare')->andReturn('SELECT ...');
@@ -60,9 +84,13 @@ class BookingRepositoryTest extends TestCase
         $row = (object) [
             'id'            => '15',
             'slot_id'       => '10',
+            'offering_id'   => null,
             'student_id'    => '5',
             'instructor_id' => '3',
+            'recurrence'    => Lesson::RECURRENCE_SINGLE,
+            'series_id'     => null,
             'status'        => 'pending',
+            'payment_id'    => null,
             'notes'         => null,
         ];
 
@@ -109,9 +137,13 @@ class BookingRepositoryTest extends TestCase
         $row = (object) [
             'id'            => '1',
             'slot_id'       => '2',
+            'offering_id'   => null,
             'student_id'    => '5',
             'instructor_id' => '3',
+            'recurrence'    => Lesson::RECURRENCE_SINGLE,
+            'series_id'     => null,
             'status'        => 'pending',
+            'payment_id'    => null,
             'notes'         => null,
         ];