Credit students for cancelled paid lessons

Cancelling a lesson that was already paid for now credits the student
that money instead of leaving it as a manual refund, and the daily
scheduled-billing scan applies any available credit against their due
charges before emailing the notice.

- New us_credits ledger + us_payments.credit_applied column (Payment::netDue).
- PaymentService::creditForCancelledLesson issues a per-lesson share of the
  covering payment's total; wired into all three cancel paths (student
  self-cancel, instructor status update, admin student-detail cancel).
- PaymentService::applyCredits draws credit down FIFO across a run's charges,
  marking a fully-covered charge paid-by-credit; the notice shows the credit
  applied and reduced total, and the admin queue shows net due.
- Student detail page shows a student's credit balance and history.

Ships as part of the unreleased 1.2.0 (same release as scheduled billing).

Tests: composer test (585), composer lint, composer cs all pass.

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

src/Payment/PaymentService.php

@@ -21,6 +21,7 @@ class PaymentService {
 		private EnrollmentRepository $enrollments,
 		private StudioSettings $settings,
 		private StripeGateway $stripe,
+		private CreditRepository $credits,
 	) {}
 
 	/**
@@ -134,6 +135,135 @@ class PaymentService {
 		}
 	}
 
+	/**
+	 * Credit a student for a cancelled lesson they had already paid for. The credit
+	 * is one lesson's share of the covering payment's total (including tax) — the
+	 * whole total for a single-lesson payment, or `total ÷ lessons covered` for a
+	 * payment that spans several (a monthly scheduled charge, or a weekly series paid
+	 * upfront). The original payment is left untouched; the credit is applied to the
+	 * student's future scheduled-billing charges. Returns null when the lesson was
+	 * never paid, has no covering payment, or was already credited.
+	 */
+	public function creditForCancelledLesson( Lesson $lesson ): ?Credit {
+		if ( null === $lesson->id ) {
+			return null;
+		}
+
+		$paymentId = $lesson->paymentId;
+		if ( null === $paymentId && null !== $lesson->seriesId ) {
+			// Series lessons other than the anchor carry no payment_id of their own;
+			// the whole reservation is paid through the anchor's payment.
+			$anchor    = $this->payments->findByRegistration( Payment::REG_LESSON, $lesson->seriesId );
+			$paymentId = $anchor?->id;
+		}
+		if ( null === $paymentId ) {
+			return null;
+		}
+
+		$payment = $this->payments->findById( $paymentId );
+		if ( null === $payment || ! $payment->isPaid() ) {
+			return null;
+		}
+
+		if ( $this->credits->existsForLesson( $lesson->id ) ) {
+			return null;
+		}
+
+		$share = round( $payment->total() / $this->coveredLessonCount( $lesson, $payment ), 2 );
+		if ( $share <= 0.0 ) {
+			return null;
+		}
+
+		$id = $this->credits->insert(
+			new Credit(
+				studentId:       $payment->studentId,
+				amount:          $share,
+				remaining:       $share,
+				currency:        $payment->currency,
+				sourcePaymentId: $payment->id,
+				sourceLessonId:  $lesson->id,
+				reason:          sprintf(
+					/* translators: %d: cancelled lesson id */
+					__( 'Credit for cancelled lesson #%d', 'unsupervised-schedular' ),
+					$lesson->id
+				),
+			)
+		);
+
+		return $this->credits->findById( $id );
+	}
+
+	/**
+	 * How many lessons the covering payment was billed for, so its total can be split
+	 * into a per-lesson credit. A weekly series paid upfront (unscheduled) covers the
+	 * whole series; every other case — a single booking, a weekly scheduled lesson
+	 * (one payment each), or a monthly scheduled charge (payment linked to each
+	 * lesson) — is answered by how many lessons point at the payment. Never below one.
+	 */
+	private function coveredLessonCount( Lesson $lesson, Payment $payment ): int {
+		if ( ! $payment->isScheduled() && null !== $lesson->seriesId ) {
+			return max( 1, $this->bookings->countBySeries( $lesson->seriesId ) );
+		}
+
+		return max( 1, $this->bookings->countByPaymentId( (int) $payment->id ) );
+	}
+
+	/**
+	 * Apply a student's available credit balance against a set of freshly-created
+	 * pending payments (the ones a billing scan just generated for them), oldest
+	 * charge first. Each payment's `credit_applied` is raised by the amount covered;
+	 * a payment fully covered is marked paid-by-credit and its registration confirmed
+	 * so it leaves the confirmation queue. The credit ledger is drawn down by the
+	 * total applied. Returns a map of payment id to the credit applied to it, so the
+	 * caller can reflect the reduction on the student's notice.
+	 *
+	 * @param list<Payment> $payments
+	 * @return array<int, float>
+	 */
+	public function applyCredits( int $studentId, array $payments ): array {
+		$balance = $this->credits->availableBalance( $studentId );
+		if ( $balance <= 0.0 ) {
+			return [];
+		}
+
+		$applied  = [];
+		$consumed = 0.0;
+
+		foreach ( $payments as $payment ) {
+			if ( null === $payment->id || $balance <= 0.0 ) {
+				continue;
+			}
+
+			$owing = $payment->netDue();
+			if ( $owing <= 0.0 ) {
+				continue;
+			}
+
+			$amount = round( min( $balance, $owing ), 2 );
+			if ( $amount <= 0.0 ) {
+				continue;
+			}
+
+			$this->payments->addCreditApplied( $payment->id, $amount );
+
+			// Fully covered by credit: settle it so it drops out of the pending queue.
+			if ( $amount >= $owing ) {
+				$this->payments->markPaid( $payment->id, 'USC-' . $payment->id );
+				$this->confirmRegistration( $payment->registrationType, $payment->registrationId );
+			}
+
+			$applied[ $payment->id ] = $amount;
+			$balance                 = round( $balance - $amount, 2 );
+			$consumed                = round( $consumed + $amount, 2 );
+		}
+
+		if ( $consumed > 0.0 ) {
+			$this->credits->consume( $studentId, $consumed );
+		}
+
+		return $applied;
+	}
+
 	/**
 	 * Resolve the client-side payment step for a freshly created registration.
 	 * For a card payment a Stripe PaymentIntent is created (or replayed