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>

src/Booking/BookingRepository.php

@@ -16,18 +16,68 @@ class BookingRepository {
 			$this->table,
 			[
 				'slot_id'       => $lesson->slotId,
+				'offering_id'   => $lesson->offeringId,
 				'student_id'    => $lesson->studentId,
 				'instructor_id' => $lesson->instructorId,
+				'recurrence'    => $lesson->recurrence,
+				'series_id'     => $lesson->seriesId,
 				'status'        => $lesson->status,
+				'payment_id'    => $lesson->paymentId,
 				'notes'         => $lesson->notes,
 				'created_at'    => current_time( 'mysql' ),
 			],
-			[ '%d', '%d', '%d', '%s', '%s', '%s' ]
+			[ '%d', '%d', '%d', '%d', '%s', '%d', '%s', '%d', '%s', '%s' ]
 		);
 
 		return $this->db->insert_id;
 	}
 
+	/**
+	 * Create a weekly lesson series — one lesson per slot, all sharing a
+	 * `series_id` (the id of the first lesson row).
+	 *
+	 * @param list<int> $slotIds Availability slot IDs to reserve, in order.
+	 * @return list<int> Inserted lesson IDs.
+	 */
+	public function insertSeries( Lesson $template, array $slotIds ): array {
+		$ids      = [];
+		$seriesId = 0;
+
+		foreach ( $slotIds as $slotId ) {
+			$id = $this->insert(
+				new Lesson(
+					slotId:       $slotId,
+					studentId:    $template->studentId,
+					instructorId: $template->instructorId,
+					offeringId:   $template->offeringId,
+					recurrence:   Lesson::RECURRENCE_WEEKLY,
+					seriesId:     $seriesId > 0 ? $seriesId : null,
+					status:       $template->status,
+					notes:        $template->notes,
+				)
+			);
+
+			if ( 0 === $seriesId ) {
+				$seriesId = $id;
+				$this->setSeriesId( $id, $seriesId );
+			}
+
+			$ids[] = $id;
+		}
+
+		return $ids;
+	}
+
+	private function setSeriesId( int $id, int $seriesId ): void {
+		$this->db->update(
+			$this->table,
+			[ 'series_id' => $seriesId ],
+			[ 'id' => $id ],
+			[ '%d' ],
+			[ '%d' ]
+		);
+	}
+
 	public function findById( int $id ): ?Lesson {
 		$row = $this->db->get_row(
 			$this->db->prepare( "SELECT * FROM {$this->table} WHERE id = %d", $id )