[performance] cached oauth database types

[NyaaaWhatsUpDoc]

Description

Updates our oauth server client + token store code to rely on our database caches.

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]

Looks good! Squerge whenever you're happy with it.

[NyaaaWhatsUpDoc]

@tsmethurst i just realized we don't have a unique constraint on our gtsmodel.Token{} "code", "refresh", "access" fields, do you think it's worth adding these?

[tsmethurst]

Mmm it's probably fine without tbh, especially since altering a table to add constraints is a bit of a faff in sqlite.