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/css/frontend.css

@@ -34,6 +34,38 @@
     margin-top: 8px;
 }
 
+.us-my-lessons {
+    margin-bottom: 24px;
+}
+
+.us-my-lesson {
+    border: 1px solid #ddd;
+    border-radius: 4px;
+    padding: 12px 16px;
+    margin-bottom: 8px;
+    display: flex;
+    justify-content: space-between;
+    align-items: center;
+}
+
+.us-lesson-status {
+    font-size: 0.85em;
+    font-weight: 600;
+    padding: 2px 10px;
+    border-radius: 10px;
+    background: #eee;
+}
+
+.us-lesson-status-confirmed {
+    background: #e2f5e5;
+    color: #1a7d2e;
+}
+
+.us-lesson-status-pending {
+    background: #fdf3d7;
+    color: #8a6d1a;
+}
+
 .us-view-toggle {
     display: flex;
     gap: 8px;