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>

docs/features/lesson-booking.md

@@ -61,12 +61,19 @@ kind `group_class`; see `group-classes.md`.
 - `[us_student_login]` — front-end login form for students
 
 ## Implementation
-- Repository: `Unsupervised\Schedular\Booking\BookingRepository`
+- Repository: `Unsupervised\Schedular\Booking\BookingRepository` (`insertSeries()` builds a weekly series sharing a `series_id`)
 - Model: `Unsupervised\Schedular\Booking\Lesson`
+- Registration gate: `Unsupervised\Schedular\Registration\RegistrationGate` — validates and records intake answers + booking-scoped policy acceptances; shared with group enrolment
 - Admin controller: `Unsupervised\Schedular\Booking\LessonController`
 - REST endpoint: `Unsupervised\Schedular\Booking\BookingEndpoint`
 - Frontend: `Unsupervised\Schedular\Booking\BookingPage`, `Unsupervised\Schedular\Auth\LoginPage`
 
+> **Payment seam:** payment is deferred to the Payments feature (#7). For now a
+> booking is created with `status = pending` and `payment_id = null`; the
+> instructor confirms via `PATCH /bookings/{id}/status`. When payments land, the
+> pay→confirm + receipt step plugs into this seam. `GET /policies?scope=booking`
+> returns just the booking-gate policies the form must collect.
+
 ## Tests
 - `tests/Unit/Booking/BookingRepositoryTest.php`
 - `tests/Unit/Booking/LessonTest.php`