chore: cleanup

[dargmuesli]

The following ideas for changes in the overall code base came up:

• use TIMESTAMPTZ, not just TIMESTAMP
• use standardized CURRENT_TIMESTAMP, not the postgres specific NOW(), see fix(timestamp): change column type timestamp to timestamptz #67
• extract the NULLIF(current_setting('jwt.claims.account_id', true), '')::UUID IS NOT NULL AND... occurrences into a separate function
• use created_at, not just created

As usual, those changes would be breaking with the current deployment style of migrations, so all changes should be released together to keep maintenance work minimal.

cc @sthelemann