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/Payment/PaymentDueMailer.php

@@ -0,0 +1,92 @@
+<?php
+declare(strict_types=1);
+
+namespace Unsupervised\Schedular\Payment;
+
+/**
+ * Emails a student a single itemised notice for every payment the daily billing
+ * scan generated for them in one run, so a student billed for several lessons on
+ * the same day receives one email with a line per item and a grand total — never
+ * one email per lesson.
+ */
+class PaymentDueMailer {
+
+	/**
+	 * Send one student their consolidated due-payment notice for the current scan.
+	 * The optional `$reference` is the shared notice-batch code the student can quote
+	 * on a lump-sum e-transfer so the studio can reconcile it to these payments.
+	 *
+	 * @param list<array{label: string, amount: float, currency: string, due_date: ?string, etransfer_email: ?string}> $items
+	 * @return bool False when there is no recipient or nothing to bill.
+	 */
+	public function send( \WP_User $student, array $items, string $reference = '' ): bool {
+		if ( '' === (string) $student->user_email || [] === $items ) {
+			return false;
+		}
+
+		$currency = (string) $items[0]['currency'];
+		$total    = 0.0;
+		$lines    = [];
+		$emails   = [];
+
+		foreach ( $items as $item ) {
+			$amount = (float) $item['amount'];
+			$total += $amount;
+
+			$lines[] = sprintf(
+				/* translators: 1: item description, 2: due date, 3: currency, 4: amount */
+				__( '- %1$s (due %2$s): %3$s %4$s', 'unsupervised-schedular' ),
+				(string) $item['label'],
+				$this->formatDate( $item['due_date'] ?? null ),
+				$currency,
+				number_format( $amount, 2 )
+			);
+
+			$etransfer = (string) ( $item['etransfer_email'] ?? '' );
+			if ( '' !== $etransfer ) {
+				$emails[ $etransfer ] = true;
+			}
+		}
+
+		$body = __( 'You have upcoming payments due:', 'unsupervised-schedular' ) . "\n\n"
+			. implode( "\n", $lines ) . "\n\n"
+			. sprintf(
+				/* translators: 1: currency, 2: total amount */
+				__( 'Total due: %1$s %2$s', 'unsupervised-schedular' ),
+				$currency,
+				number_format( $total, 2 )
+			);
+
+		if ( [] !== $emails ) {
+			$body .= "\n\n" . sprintf(
+				/* translators: %s: e-transfer destination email address(es) */
+				__( 'Please send your e-transfer to: %s', 'unsupervised-schedular' ),
+				implode( ', ', array_keys( $emails ) )
+			);
+		}
+
+		if ( '' !== $reference ) {
+			$body .= "\n\n" . sprintf(
+				/* translators: %s: payment reference code */
+				__( 'Please include this reference with your payment: %s', 'unsupervised-schedular' ),
+				$reference
+			);
+		}
+
+		return (bool) wp_mail( $student->user_email, __( 'Payment due', 'unsupervised-schedular' ), $body );
+	}
+
+	/**
+	 * Present a stored `Y-m-d` due date in a friendlier form; falls back to the
+	 * raw value (or an empty string) when it is not a parseable date.
+	 */
+	private function formatDate( ?string $date ): string {
+		if ( null === $date || '' === $date ) {
+			return '';
+		}
+
+		$parsed = \DateTimeImmutable::createFromFormat( '!Y-m-d', $date );
+
+		return false !== $parsed ? $parsed->format( 'M j, Y' ) : $date;
+	}
+}