user: select ticket options and lots quantity click on "buy now"
system receive request with (user-id, event-id, ticket-opt-id, ticket-opt-lot-id) validate if the event isn't sold-out
if event not sold-out ...
reduce 1 ticket of the available amount (reserve ticket for 10 min)
user go to checkout
await successful event from stripe ...
create a new ticket in ticket's service (... all required info)