Update a subscription's next billing cycle

Create a subscription for a client against one of your billing periods. `client.phone` identifies the buyer and must be a Nepali mobile number (10 digits starting with 96, 97 or 98); a +977 country code is accepted and stripped. The subscription starts in `pending_checkout`; send the returned `checkout_url` to the client to collect payment. `client.email` and `client.phone` are read back on the subscription as `client.email` and `client.phone`, but the `client.billing` block loses its `billing_` prefix on the way back out. `return_url` is required despite reading as optional on the model behind this body — a missing, null or blank value is rejected the same way. A client may hold only one live recurring subscription per product, so a second one on a different billing period of the same product is refused. This endpoint takes no idempotency key: a retried request that timed out opens a second subscription and a second checkout, so reconcile by `subscription_id` before retrying.