[performance] cache v2 filter keyword regular expressions

[NyaaaWhatsUpDoc]

• [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.
• [ ] I/we have made any necessary changes to documentation.
• [ ] 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.

[tsmethurst]

Looks sensible on first glance, I'll review properly later though :)

[VyrCossont]

Looks good to me. Thanks for the perf boost!