List a newsletter's sends

Every dispatch this newsletter has made, most recent first, with the numbers each one finished on. Needs sending: read.

A send is one run rather than one article. An article that went out once has one; an article that failed and was sent again has two, and ?article= narrows the collection to one article's runs. An article imported from an outside provider has none, because that provider mailed it, and neither has an article whose audience worked out to nobody.

The three counts are the three a send.completed event publishes, under the same names, so a consumer holding a stored event and a caller reading this back are holding one set of numbers rather than two spellings of them. Send says what each one counts and what none of them counts.

Expanding article inlines an Article, which is content rather than sending, so it also needs content: read on this newsletter. A credential that holds sending: read without it is refused with 403 insufficient_scope, param set to expand, and allowed_values listing what it may expand instead (newsletter). Unexpanded, article is a reference and needs nothing more. Accepted paths are article and newsletter.