Migration: Increase the precision of subscription timestamp fields

[RobertJoonas]

Changes

Motivation: Paddle sometimes sends us two webhooks at the same time when a subscription is updated. We only need to count the latest one of those, for which we need more precision than 1s.

The migration will set both inserted_at and updated_at columns in the subscriptions table to use the :naive_datetime_usec type instead of just :naive_datetime. In practice, we currently need more precision only on the updated_at field. But I suppose it's better to keep the type between both timestamp fields consistent.

Tests

• [x] Automated tests have been added
• [ ] This PR does not require tests

Changelog

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• [x] This PR does not make a user-facing change

Documentation

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Dark mode

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• [x] This PR does not change the UI