[chore] set max open / idle conns + conn max lifetime for both postgres and sqlite

[tsmethurst]

Description

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This PR updates some of our database tuning. Previously we were only setting maxopenconns and maxidleconns for postgres, and only setting connmaxlifetime(0) for in-mem sqlite connections, to avoid the db being deleted on connection cleanup.

Now, this tuning has been combined and is done for both database types. This is partially because we should have already been setting maxopenconns and maxidleconns for sqlite but weren't, and also to see if some SQLITE_BUSY errors can be resolved by never letting sqlite connections die (see https://www.sqlite.org/wal.html#sometimes_queries_return_sqlite_busy_in_wal_mode)

The PR also reduces the default cache size for sqlite connections to 8MiB, since 64MiB ended up using rather a lot of memory.

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.

[tsmethurst]

This seems pretty decent now so i'm gonna merge and we can still play around with some of the values on main as necessary! :)