[bugfix] fix inconsistent calculated cache sizes

[NyaaaWhatsUpDoc]

• use pre-calculated exampleTime instead of time.Now() to ensure no locale data in example models
• tweaks the cache memory ratios now the calculated sizes of changed
• updates the default cache memory target to 100MiB, given the previous calculated sizes were smaller than they should be

for a more full explanation see the comment on var exampleTime in internal/cache/size.go

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 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.

[NyaaaWhatsUpDoc]

the psychic damage tracking down this bug took years off my life

[daenney]

Time should've never been invented.