Apply account credit on same-month rebook, surface credit on student page

A monthly/weekly lesson booked into a month whose billing date has
already passed is charged at booking time by LessonBooker::settle,
bypassing the daily scan where account credit is otherwise applied. So
rebooking a cancelled paid lesson within the same month charged the
family in full while their cancellation credit sat unused — billed twice
for the same slot.

Apply the payer's credit to that charge-at-booking payment for scheduled
offerings, mirroring the daily scan: a payment fully covered by credit
settles and confirms its lesson. Add PaymentService::findPayment so
settle can re-read the row after applyCredits writes to it.

Also surface the student's total account credit at the top of their
detail page when they hold a balance, so the studio sees it at a glance.

Co-authored-by: anthropic/claude-opus-4-8

src/Booking/LessonBooker.php

@@ -184,6 +184,7 @@ class LessonBooker {
 				? $offering->price
 				: $offering->price * count( $ids );
 
+			$payerId = $this->guardians->payerFor( $studentId );
 			$payment = $this->payments->createForRegistration(
 				Payment::REG_LESSON,
 				$anchorId,
@@ -192,9 +193,23 @@ class LessonBooker {
 				$amount,
 				$offering->currency,
 				$offering->etransferEmail,
-				payerId: $this->guardians->payerFor( $studentId )
+				payerId: $payerId
 			);
 
+			// A scheduled lesson charged at booking — the extra lesson added to (or a
+			// cancelled one rebooked within) an already-billed month — skips the daily
+			// scan, which is the only place credit is otherwise applied. Apply the
+			// payer's account credit here too, so a cancellation credit settles the
+			// rebooking instead of the family being charged twice for the same slot. A
+			// pending payment fully covered by credit is settled and its lesson
+			// confirmed; applyCredits is a no-op when the payer holds no balance. The
+			// one-time pay-now flow is left alone: its payment step already fronts the
+			// student, and credit there is a manual studio decision.
+			if ( $offering->isScheduledBilling() && null !== $payment && null !== $payment->id && Payment::STATUS_PENDING === $payment->status ) {
+				$this->payments->applyCredits( $payerId, [ $payment ] );
+				$payment = $this->payments->findPayment( (int) $payment->id ) ?? $payment;
+			}
+
 			return [
 				'status'  => null !== $payment && $payment->isPaid() ? Lesson::STATUS_CONFIRMED : Lesson::STATUS_PENDING,
 				'payment' => $payment,

src/Payment/PaymentService.php

@@ -116,6 +116,15 @@ class PaymentService {
 		return $this->payments->markNoticed( $paymentId );
 	}
 
+	/**
+	 * Re-read a payment from the ledger — the caller's way to pick up a status or
+	 * credit change {@see applyCredits} wrote straight to the row, since the
+	 * Payment object it holds is immutable. Delegates to the ledger.
+	 */
+	public function findPayment( int $paymentId ): ?Payment {
+		return $this->payments->findById( $paymentId );
+	}
+
 	/**
 	 * Studio-admin confirmation that a pending payment (e-transfer) was received.
 	 * Marks it paid, confirms the registration, and emails the receipt.

templates/admin/student-detail.php

@@ -114,6 +114,39 @@ $renderLessons = static function (array $rows, bool $withActions = false): void
         <div class="notice notice-error is-dismissible"><p><?php echo esc_html($error); ?></p></div>
     <?php endif; ?>
 
+    <?php
+    /*
+     * Surface the credit balance up top the moment there is one, so the studio
+     * sees at a glance that this student is owed against future billing without
+     * scrolling to the Account credit section. Only shown when there is credit to
+     * report — a zero balance is not news. The full breakdown stays below.
+     */
+    ?>
+    <?php if ($canBilling && $creditBalance > 0) : ?>
+        <div class="notice notice-info inline">
+            <p>
+                <?php
+                printf(
+                    /* translators: %s: total available credit, e.g. "45.00 CAD" */
+                    esc_html__('Total account credit: %s', 'unsupervised-schedular'),
+                    '<strong>' . esc_html(number_format_i18n($creditBalance, 2) . ' ' . $creditCurrency) . '</strong>'
+                );
+                ?>
+                <?php if ($payer['id'] !== (int) $student->ID) : ?>
+                    <span class="description">
+                        <?php
+                        printf(
+                            /* translators: %s: name of the parent/guardian whose account holds the balance. */
+                            esc_html__('Held on %s’s account.', 'unsupervised-schedular'),
+                            esc_html($payer['name'])
+                        );
+                        ?>
+                    </span>
+                <?php endif; ?>
+            </p>
+        </div>
+    <?php endif; ?>
+
     <?php $detailUrl = static fn(int $id): string => add_query_arg(['page' => $pageSlug, 'student_id' => $id], admin_url('admin.php')); ?>
 
     <h2><?php esc_html_e('Account', 'unsupervised-schedular'); ?></h2>

tests/Unit/Booking/BookingEndpointTest.php

@@ -57,6 +57,13 @@ class BookingEndpointTest extends TestCase
         // Crediting a cancelled paid lesson is exercised in dedicated tests; other
         // cancellation paths simply allow the call.
         $this->payments->shouldReceive('creditForCancelledLesson')->andReturn(null)->byDefault();
+        // A charge-at-booking scheduled lesson (an add-on to an already-billed month)
+        // applies the payer's credit before it settles. The default holds no balance
+        // and re-reads the same payment; the same-month-rebook test overrides both.
+        $this->payments->shouldReceive('applyCredits')->andReturn([])->byDefault();
+        $this->payments->shouldReceive('findPayment')->andReturnUsing(
+            static fn (int $id): ?Payment => null
+        )->byDefault();
 
         $this->guardians = Mockery::mock(GuardianService::class);
         // The default account books only for itself: no guardian link anywhere.
@@ -463,6 +470,42 @@ class BookingEndpointTest extends TestCase
         self::assertNotNull($result->get_data()['payment']);
     }
 
+    public function testMonthlyRebookInBilledMonthAppliesAccountCredit(): void
+    {
+        // Rebooking a cancelled monthly lesson inside an already-billed month is
+        // charged at booking. The payer holds a cancellation credit that must be
+        // applied to that charge — otherwise the family is billed twice for the same
+        // slot. A credit that fully covers it settles the payment and confirms the
+        // lesson, so the front end runs no payment step.
+        $this->availability->shouldReceive('findById')->with(10)->andReturn(
+            new AvailabilitySlot(instructorId: 3, startDt: '2026-06-20 10:00:00', endDt: '2026-06-20 11:00:00', offeringId: null, id: 10)
+        );
+        $this->offerings->shouldReceive('findById')->with(8)->andReturn(
+            new Offering(instructorId: 3, kind: Offering::KIND_PRIVATE_LESSON, title: 'Lesson', price: 45.0, billingMode: Offering::BILLING_MONTHLY, id: 8)
+        );
+        $this->gate->shouldReceive('validate')->andReturn(null);
+        $this->availability->shouldReceive('claim')->with(10)->once()->andReturn(true);
+        $this->bookings->shouldReceive('insert')->once()->andReturn(77);
+        $this->gate->shouldReceive('record')->once();
+
+        $pending = new Payment(5, 3, Payment::REG_LESSON, 77, 45.0, currency: 'CAD', method: Payment::METHOD_ETRANSFER, status: Payment::STATUS_PENDING, id: 12);
+        $this->payments->shouldReceive('createForRegistration')
+            ->once()
+            ->with(Payment::REG_LESSON, 77, 5, 3, 45.0, 'CAD', null, null, null, 5)
+            ->andReturn($pending);
+
+        // Credit is applied against the fresh charge for the same payer.
+        $this->payments->shouldReceive('applyCredits')->once()->with(5, [$pending])->andReturn([12 => 45.0]);
+        // applyCredits settled the row; the re-read reflects it as paid.
+        $paid = new Payment(5, 3, Payment::REG_LESSON, 77, 45.0, currency: 'CAD', method: Payment::METHOD_ETRANSFER, status: Payment::STATUS_PAID, creditApplied: 45.0, id: 12);
+        $this->payments->shouldReceive('findPayment')->with(12)->andReturn($paid);
+
+        $result = $this->endpoint->book(new \WP_REST_Request(['slot_id' => 10, 'offering_id' => 8]));
+
+        self::assertInstanceOf(\WP_REST_Response::class, $result);
+        self::assertSame(Lesson::STATUS_CONFIRMED, $result->get_data()['status']);
+    }
+
     public function testMonthlyLessonBeforeBillingDateDefersPayment(): void
     {
         // "now" is 2026-06-01; a monthly lesson for July is booked before July's 1st,