Apply available credit to any at-booking charge, not just scheduled

A payer's account credit should offset any charge raised at booking, so
drop the scheduled-only guard. Covers the one-time pay-now flow too.

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

tests/Unit/Booking/BookingEndpointTest.php

@@ -57,9 +57,9 @@ 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.
+        // A charge raised at booking 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