DB: support "best price search"

[derhuerst]

@bergmannjg I went ahead and created a WIP PR.

Fixes #291.
Fixes juliuste/db-prices#33.
Fixes derhuerst/db-prices-cli#4.

[derhuerst]

Just a heads-up: There might be a case – that I can't think of right now though – that fails because transformJourneysQuery() assumes a TripSearch call.

[derhuerst]

Sorry, even though I told you to do it this way, I think this isn't correct either: For example, if profile.timezone is Europe/Berlin, the timezone-naive now is 2024-05-06T00:30+01:00 because it runs on a machine in Europe/London, then this will create a Berlin-timezone timestamp for the next day.

I can think of a least two options, the latter being the more straightforward one IMO:

1. Change now to be timezone-aware, so that now.{year, month,day} are correct.
2. Just use one chain of DateTime calls, as illustrated below.

Also, while implicitly casting today to a number below works, it would be cleaner to add .toMillis().

const today = DateTime
	.fromMillis(Date.now(), {
		zone: profile.timezone,
		locale: profile.locale,
	})
	.startOf('day')
	.toMillis()

[derhuerst]

remotely related: #331