[chore] Use 'immediate' lock for sqlite transactions

[tsmethurst]

Description

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If this is a documentation change, please briefly describe what you've changed and why.

This pull request updates sqlite preferences to use 'immediate' locking mode for transactions instead of the default 'deferred' mode. This won't make much of a difference right now, but should be useful in future when we implement proper retry behavior on SQLITE_BUSY errors.

Also tidies up some of the sqlite preferences stuff a little bit, and adds some explanatory comments for why we do certain things.

Checklist

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• 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. n/a
• I/we have run tests and they pass locally with the changes.
• I/we have run go fmt ./... and golangci-lint run.