[bugfix] Return 501 (not implemented) if user tries to schedule post

[tsmethurst]

Description

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This PR updates status validation code to return 501 if a status is submitted using "scheduled_at", instead of just publishing the status as a normal, immediate status. The latter behavior was causing issues with clients: tuskyapp/Tusky#4703

Also does a bit of gentle cleaning up in the status create file.

Relates to #1006

Checklist

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