Add a per-ticket booking fee to the price shown and charged

Ticketing operators commonly add a booking or handling fee on top of the ticket price, shown beside it at selection time ("+ booking fee") and included in what is charged. yoyaku has no such concept: a line's amount comes from the payment policy resolver, and the only fee it models is the per-resource no-show fee, charged after the event through a workflow action, which is a different thing entirely.

Adding one is not just a display change, so the model needs deciding first:

• Where is it configured? Site-wide, per resource beside the other payment settings, or per tariff, so that a subscription can be exempt while a single ticket is not.
• Per ticket or per order? Both exist in the wild, and the answer decides whether it belongs on a line or on the transaction.
• Is it refundable? Operators frequently refund the ticket and keep the fee, which means it cannot simply be folded into the line amount.
• Does it show on the ticket? And in the order summary, as its own row or as part of the total.
• Fixed, percentage, or banded? A flat amount per ticket is the common case; a percentage interacts with the tax handling.

Until that is settled, the booking page should keep showing the price alone. The layout work in #3614127: Reshape the slot booking page: a flat column of quantity spinners with price as helper text assumes exactly that, so this can land independently either before or after it.