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/Schema.php

@@ -32,15 +32,21 @@ class Schema {
 			"CREATE TABLE {$prefix}us_lessons (
                 id            BIGINT UNSIGNED NOT NULL AUTO_INCREMENT,
                 slot_id       BIGINT UNSIGNED NOT NULL,
+                offering_id   BIGINT UNSIGNED DEFAULT NULL,
                 student_id    BIGINT UNSIGNED NOT NULL,
                 instructor_id BIGINT UNSIGNED NOT NULL,
+                recurrence    VARCHAR(10)     NOT NULL DEFAULT 'single',
+                series_id     BIGINT UNSIGNED DEFAULT NULL,
                 status        VARCHAR(20)     NOT NULL DEFAULT 'pending',
+                payment_id    BIGINT UNSIGNED DEFAULT NULL,
                 notes         TEXT,
                 created_at    DATETIME        NOT NULL,
                 PRIMARY KEY (id),
                 KEY slot_id (slot_id),
+                KEY offering_id (offering_id),
                 KEY student_id (student_id),
-                KEY instructor_id (instructor_id)
+                KEY instructor_id (instructor_id),
+                KEY series_id (series_id)
             ) {$charset};",
 
 			"CREATE TABLE {$prefix}us_offerings (