Skip payment step for unpriced bookings, confirm them immediately, show students their lessons

Booking a slot with no priced offering created the lesson but no payment,
yet the front end still called POST /payments/intent, which 400ed with
"Could not start payment for this registration" — the student saw an error
while the backend held a claimed slot and a lesson stuck at pending.

- POST /bookings and POST /enrollments now return a `payment` summary
  ({id, method, status}) or null when nothing is owed; the JS only runs
  the payment step when a payment exists.
- Bookings with nothing owed are confirmed at creation — there is no
  payment step that would ever confirm them later.
- The booking page now shows the student's upcoming lessons (GET /bookings,
  now scoped to upcoming non-cancelled lessons with slot start/end times)
  with a pending-payment/confirmed status badge.

Fixes #53

Co-Authored-By: Claude Fable 5 <[email protected]>

tests/Unit/Booking/BookingRepositoryTest.php

@@ -132,6 +132,35 @@ class BookingRepositoryTest extends TestCase
         self::assertFalse($this->repo->updateStatus(1, Lesson::STATUS_CONFIRMED));
     }
 
+    public function testFindUpcomingForStudentJoinsSlotAndExcludesCancelled(): void
+    {
+        Functions\when('current_time')->justReturn('2026-06-08 12:00:00');
+
+        $this->db->shouldReceive('prepare')
+            ->once()
+            ->with(Mockery::pattern('/SELECT l\.\*.*l.student_id = %d.*l.status != %s.*a.start_dt >= %s.*ORDER BY a.start_dt ASC/s'), 'wp_us_lessons', 'wp_us_availability', 5, Lesson::STATUS_CANCELLED, '2026-06-08 12:00:00')
+            ->andReturn('SELECT ...');
+
+        $row = (object) [
+            'id'            => '15',
+            'slot_id'       => '10',
+            'offering_id'   => null,
+            'student_id'    => '5',
+            'instructor_id' => '3',
+            'recurrence'    => Lesson::RECURRENCE_SINGLE,
+            'series_id'     => null,
+            'status'        => 'confirmed',
+            'payment_id'    => null,
+            'notes'         => null,
+        ];
+        $this->db->shouldReceive('get_results')->andReturn([$row]);
+
+        $lessons = $this->repo->findUpcomingForStudent(5);
+
+        self::assertCount(1, $lessons);
+        self::assertSame(15, $lessons[0]->id);
+    }
+
     public function testCountUpcomingForStudent(): void
     {
         Functions\when('current_time')->justReturn('2026-06-08 12:00:00');