Reset subscription max_seats_used on trial upgrade

What does this MR do and why?

Follow-up from !110768 (merged) to ensure we're resetting seats for trial upgrades.

When a customer upgrades from a trial to a paid plan on the same day they started the trial, previously the max seats would not be reset, but there are edge cases where we'd want to do this (e.g. the customer adds users during the trial, but removes before upgrade).

This MR adjusts the logic in two ways:

1. caters for upgrades to trials on the same day, as above
2. only resets seats for paid plan subscriptions - this matches the logic used for the daily refresh of statistics

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