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]>

docs/features/payments.md

@@ -94,7 +94,7 @@ After booking, the destination on a payment can be corrected per booking:
 | `paid_at`                  | DATETIME         | When marked `paid`; NULL otherwise                    |
 
 ## Payment Flow
-1. During registration the front-end calls `POST /payments/intent`, which creates a Stripe PaymentIntent for a `card` student and returns the client secret. (`etransfer` returns a `pending` payment; `comp` returns none.)
+1. During registration the front-end calls `POST /payments/intent` — but only when the registration response carried a `payment` summary (unpriced registrations return `payment: null` and skip the payment step). The intent call creates a Stripe PaymentIntent for a `card` student and returns the client secret. (`etransfer` returns a `pending` payment; `comp` returns none.)
 2. The browser confirms the card payment with Stripe.
 3. Stripe calls `POST /payments/webhook`; on `payment_intent.succeeded` the payment is marked `paid`, `paid_at` is stamped, and the linked lesson/enrolment is `confirmed`.
 4. On transition to `paid`, `ReceiptMailer` assigns a `receipt_number`, emails the student a receipt, and stamps `receipt_sent_at`.