Add created_at to events table

[klippx]

There is currently only one date on events: event_time. This is handed over to the user/host of phobos db checkpoint, and not mandatory to set. This means sometimes/often events will not have event_time set at all.

Since the events api will return events sorted by event_time: :desc, the events will appear first even if they are really old (according to metadata) so that gives a bad experience in PhobosCheckpointUI.

Therefor I propose we add created_at to events table, and sort primarily by event_time and secondarily by created_at.

This column would also provide quite useful information about events when poking around.