* Make subsequent query boundaries idempotent

This should prevent time-sensitive race conditions
on subsequent `Plausible.Stats.*` calls for the same query.
Not sure if there are more places where now() is
dynamic relative to query life cycle, but it's a start.

* Format

* Update lib/plausible/stats/query.ex

Co-authored-by: Wojtek Mach <wojtekmach@users.noreply.github.com>

* Match on `Query.now` in a test

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Co-authored-by: Wojtek Mach <wojtekmach@users.noreply.github.com>