[bugfix] Hoist filterable text field extraction out of loop

[VyrCossont]

Description

This doesn't change within the loop, so there's no point in repeating it. This should make filter processing faster in the common case where a user has more than one applicable filter.

Discovered while investigating #3128

Checklist

• [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.