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

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,