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/bootstrap.php

@@ -13,3 +13,36 @@ define('USC_VERSION', '1.0.0');
 define('USC_PLUGIN_FILE', dirname(__DIR__) . '/unsupervised-schedular.php');
 define('USC_PLUGIN_DIR', dirname(__DIR__) . '/');
 define('USC_PLUGIN_URL', 'http://example.com/wp-content/plugins/unsupervised-schedular/');
+
+// Minimal WP_Error stub for code under test that returns error objects.
+if (! class_exists('WP_Error')) {
+    class WP_Error
+    {
+        /** @var array<string, list<string>> */
+        public array $errors = [];
+
+        /** @var array<string, mixed> */
+        public array $error_data = [];
+
+        public function __construct(string $code = '', string $message = '', mixed $data = '')
+        {
+            if ('' !== $code) {
+                $this->errors[$code][] = $message;
+                if ('' !== $data) {
+                    $this->error_data[$code] = $data;
+                }
+            }
+        }
+
+        public function get_error_code(): string
+        {
+            return (string) (array_key_first($this->errors) ?? '');
+        }
+
+        public function get_error_message(): string
+        {
+            $code = $this->get_error_code();
+            return '' !== $code ? ($this->errors[$code][0] ?? '') : '';
+        }
+    }
+}