[chore] status dereferencing improvements

[NyaaaWhatsUpDoc]

Description

• in the case that mention only provides a namestring, search for existing mention by username,domain
• handle definitely deleted statuses (i.e. they return 401) in the dereferencer by deleting them

Checklist

• 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.

[tsmethurst]

Seems reasonable :) Just one small comment but not an important one, squerge away when you're happy.