[bugfix] Fix incorrect field name for status source, add helpful message

[tsmethurst]

Description

If this is a code change, please include a summary of what you've coded, and link to the issue(s) it closes/implements.

If this is a documentation change, please briefly describe what you've changed and why.

I misnamed a field name in the status source PR, so this fixes that. Also adds a helpful message returning status source (which is used for edits), indicating to the user that they won't be able to resubmit the changed source as an edit.

The little message shown to the user after they've clicked "edit" and they get the original post text open in their client looks like this:

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.

[NyaaaWhatsUpDoc]

what is this swagger.yam

where can i get some yam

[tsmethurst]

You rascal. Fixed it!

[NyaaaWhatsUpDoc]

You rascal. Fixed it!

oh I'M the rascal here, huh??

[tsmethurst]

Confirmed rascal by a jury of your peers.