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/Availability/AvailabilityRepository.php

@@ -141,6 +141,22 @@ class AvailabilityRepository {
 		return array_map( AvailabilitySlot::fromRow( ... ), $rows ?? [] );
 	}
 
+	/**
+	 * Unbooked slots that belong to a weekly-recurring group, ordered by start.
+	 *
+	 * @return list<AvailabilitySlot>
+	 */
+	public function findUnbookedInGroup( int $recurrenceGroup ): array {
+		$rows = $this->db->get_results(
+			$this->db->prepare(
+				"SELECT * FROM {$this->table} WHERE recurrence_group = %d AND is_booked = 0 ORDER BY start_dt ASC",
+				$recurrenceGroup
+			)
+		);
+
+		return array_map( AvailabilitySlot::fromRow( ... ), $rows ?? [] );
+	}
+
 	public function findById( int $id ): ?AvailabilitySlot {
 		$row = $this->db->get_row(
 			$this->db->prepare( "SELECT * FROM {$this->table} WHERE id = %d", $id )