[bugfix] Extract description as summary first, fall back to name

[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.

Honk provides both name and summary in their attachments. Unlike many other AP implementations, they actually use these correctly, so "name" is the name of the file, and "summary" is the image description. Previously, we were only checking the "name" field to parse image description, since this is what most other implementations use. This PR fixes this to check "summary" first and fall back to "name".

Closes #2302

Checklist

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If this is a documentation change, only the first checkbox must be filled (you can delete the others if you want).

• 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 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.