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/Payment/Payment.php

@@ -84,6 +84,20 @@ class Payment {
 		return round( $this->amount + $this->taxAmount, 2 );
 	}
 
+	/**
+	 * Minimal payment info embedded in registration-creation responses: enough
+	 * for the front end to decide whether (and how) to run the payment step.
+	 *
+	 * @return array<string, mixed>
+	 */
+	public function toSummaryArray(): array {
+		return [
+			'id'     => $this->id,
+			'method' => $this->method,
+			'status' => $this->status,
+		];
+	}
+
 	/**
 	 * Returns a plain array representation of the payment.
 	 *