Add weekly and monthly scheduled billing for offerings

Offerings can now bill weekly (a pending payment 24h before each lesson)
or monthly (one payment on the 1st for that month's lessons), alongside
one-time and full-term. Applies to both private lessons and group classes.

- Offering: new `weekly`/`monthly` billing modes + `isScheduledBilling()`
- Booking/enrolment defer payment for scheduled modes; a single lesson
  booked after its due date has passed (e.g. an add-on in an already-billed
  month) is charged at booking instead
- ScheduledBillingRunner: daily WP-Cron scan generates due payments across
  four cases (private/group × weekly/monthly), deduped via lesson.payment_id
  and payments.period_key
- PaymentDueMailer: one consolidated itemised email per student per scan
- Notice batch: payments emailed together share a reference; the admin
  Payments queue groups them with a lump-sum total for e-transfer reconciliation
- Cancellation never voids a scheduled payment (Payment::isScheduled())
- Schema: us_payments gains due_date, period_key, notice_batch; USC_VERSION 1.2.0

composer test, composer lint, composer cs all pass.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

tests/Unit/GroupClass/EnrollmentEndpointTest.php

@@ -109,6 +109,22 @@ class EnrollmentEndpointTest extends TestCase
         );
     }
 
+    public function testScheduledBillingEnrollmentDefersPayment(): void
+    {
+        // A monthly group class is billed later by the daily scan, so enrolment
+        // succeeds with no payment created now.
+        $offering = new Offering(instructorId: 3, kind: Offering::KIND_GROUP_CLASS, title: 'Choir', price: 120.0, billingMode: Offering::BILLING_MONTHLY, id: 8);
+        $this->offerings->shouldReceive('findById')->with(8)->andReturn($offering);
+        $this->expectSuccessfulEnrollment();
+        $this->payments->shouldNotReceive('createForRegistration');
+
+        $result = $this->endpoint->enroll(new \WP_REST_Request(['offering_id' => 8]));
+
+        self::assertInstanceOf(\WP_REST_Response::class, $result);
+        self::assertSame(201, $result->get_status());
+        self::assertNull($result->get_data()['payment']);
+    }
+
     public function testRejectsEnrollmentAfterExplicitDeadline(): void
     {
         // current_time is stubbed to 2026-07-24, past the 2026-07-10 deadline.