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]>

assets/js/group-classes.js

@@ -142,7 +142,11 @@
                 accepted_policy_version_ids: accepted,
             }),
         })
-            .then((res) => window.usPayment.collect('enrollment', res.id, list))
+            // An enrolment with nothing owed has no payment, so there is no
+            // payment step to run.
+            .then((res) => (res.payment
+                ? window.usPayment.collect('enrollment', res.id, list)
+                : null))
             .then((result) => showConfirmation(window.usPayment.message(result)))
             .catch((err) => showError(err.message));
     }