[bugfix] Account.last_status_at is a date, not datetime

[untitaker]

Description

Changes API Account's last_status_at to be a date without time component, for full compat with mastodon.

closes #3418

Checklist

Please put an x inside each checkbox to indicate that you've read and followed it: [ ] -> [x]

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• [x] I/we have read the GoToSocial contribution guidelines.
• [x] I/we have discussed the proposed changes already, either in an issue on the repository, or in the Matrix chat.
• [x] I/we have not leveraged AI to create the proposed changes.
• [x] I/we have performed a self-review of added code.
• [x] I/we have written code that is legible and maintainable by others.
• [x] I/we have commented the added code, particularly in hard-to-understand areas.
• [x] I/we have made any necessary changes to documentation.
• [x] I/we have added tests that cover new code.
• [x] I/we have run tests and they pass locally with the changes.
• [x] I/we have run go fmt ./... and golangci-lint run.

[untitaker]

~converting to draft until we know all tests pass. only ran internal/typeutils/, the rest is too resource-intensive on local hardware.~

golangci-lint run does not pass for me on main branch, but this PR does not add new errors. the link in CONTRIBUTING for the installation instructions is out of date, and I suspect I installed too new a version that comes with additional lints: golangci-lint has version 1.61.0 built with go1.23.1 from a1d6c560 on 2024-09-09T17:44:42Z

[tsmethurst]

Thanks! ^_^