Change default events API behavior when startdate and/or enddate aren't specified

We recommend clients always make events API requests for a single event by id, or a range of dates with the startdate and enddate parameters. If an id is provided, it takes precedence; if not, we look for a range.

If the client doesn't specify them, startdate and enddate both default to the current date:

• Providing only startdate often returns an error since enddate is implied as today; any future startdate, or a past startdate more than 100 days ago, will return a 400 error
• If only enddate is provided and it's before the current date or more than 100 days from the current date, a 400 error is returned.
• If neither is provided, events for the current date are returned (1 day range)

Proposed: startdate defaults to current date; enddate defaults to startdate +9 days (= 10 day range)

• Providing only startdate always gives you 10 days of events (current date + 9 days, which is what the /calendar/ list view currently shows by default), regardless of how far in the past or future
• Same enddate error cases
• If neither is provided, events for the current date +9 days (= 10 day range) are returned
• pagination object in response would be explicit about what implicit range was actually returned (see #552)

Start by locating the events API implementation and comparing its defaults with the /calendar/ list view behavior; review issue #552 for the pagination response requirement. Done means omitted dates produce the proposed 10-day range, explicit enddate error cases remain unchanged, and the response pagination object reports the implicit range.