fix(ri-exchange): run ExecuteExchange outside the request context #790

RI exchange ExecuteExchange runs on the HTTP request context (internal/api/handler_ri_exchange.go:1921). A client disconnect or the 30s handler timeout can cancel it after the provider exchange committed, same class as #706 (fixed for purchases in #789). Detach the execution and its terminal save the same way, with their own timeouts.