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]>

src/Booking/BookingRepository.php

@@ -204,6 +204,45 @@ class BookingRepository {
 		return array_map( Lesson::fromRow( ... ), $rows ?? [] );
 	}
 
+	/**
+	 * Not-yet-billed lessons on a scheduled-billing (weekly / monthly) offering:
+	 * status not cancelled and no payment attached yet. Each row carries the slot
+	 * start time and the offering's billing fields so the daily billing scan can
+	 * decide what is due without a second query per lesson. Ordered by student,
+	 * offering and time so the scan can group a student's monthly lessons cheaply.
+	 *
+	 * @return list<\stdClass> Rows: id, student_id, instructor_id, offering_id,
+	 *                         start_dt, billing_mode, title, price, currency,
+	 *                         etransfer_email.
+	 */
+	public function findUnbilledScheduledLessons(): array {
+		$avTable  = str_replace( 'us_lessons', 'us_availability', $this->table );
+		$offTable = str_replace( 'us_lessons', 'us_offerings', $this->table );
+
+		$rows = $this->db->get_results(
+			$this->db->prepare(
+				'SELECT l.id, l.student_id, l.instructor_id, l.offering_id,
+				        a.start_dt,
+				        o.billing_mode, o.title, o.price, o.currency, o.etransfer_email
+				 FROM %i l
+				 JOIN %i a ON a.id = l.slot_id
+				 JOIN %i o ON o.id = l.offering_id
+				 WHERE l.status != %s
+				   AND l.payment_id IS NULL
+				   AND o.billing_mode IN ( %s, %s )
+				 ORDER BY l.student_id ASC, l.offering_id ASC, a.start_dt ASC',
+				$this->table,
+				$avTable,
+				$offTable,
+				Lesson::STATUS_CANCELLED,
+				\Unsupervised\Schedular\Offering\Offering::BILLING_WEEKLY,
+				\Unsupervised\Schedular\Offering\Offering::BILLING_MONTHLY
+			)
+		);
+
+		return $rows ?? [];
+	}
+
 	public function setPaymentId( int $id, int $paymentId ): bool {
 		return false !== $this->db->update(
 			$this->table,