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]>

src/Booking/BookingRepository.php

@@ -113,6 +113,33 @@ class BookingRepository {
 		return array_map( Lesson::fromRow( ... ), $rows ?? [] );
 	}
 
+	/**
+	 * Upcoming lessons for a student (status != cancelled, slot in the future).
+	 *
+	 * @return list<Lesson>
+	 */
+	public function findUpcomingForStudent( int $studentId ): array {
+		$avTable = str_replace( 'us_lessons', 'us_availability', $this->table );
+
+		$rows = $this->db->get_results(
+			$this->db->prepare(
+				'SELECT l.* FROM %i l
+                 JOIN %i a ON a.id = l.slot_id
+                 WHERE l.student_id = %d
+                   AND l.status != %s
+                   AND a.start_dt >= %s
+                 ORDER BY a.start_dt ASC',
+				$this->table,
+				$avTable,
+				$studentId,
+				Lesson::STATUS_CANCELLED,
+				current_time( 'mysql' )
+			)
+		);
+
+		return array_map( Lesson::fromRow( ... ), $rows ?? [] );
+	}
+
 	/**
 	 * Count a student's upcoming, non-cancelled lessons (slot in the future).
 	 */