[bugfix] Fix temp table deletion causing runaway allocations

[tsmethurst]

Description

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This pull request addresses an issue we've been having with runaway memory allocations causing stress on the garbage collector. For some reason, deleting temporary tables causes SQLite to flip out and keep allocating memory, which is then immediately cleaned up.

This PR fixes the issue by changing the way we delete temporary tables: for Postgres, we can just use ON COMMIT DROP, which causes the temp tables to be cleaned up when the transaction ends. For SQLite, we can adjust the max conn lifetime to 5 minutes to ensure that temp tables get closed when the connection is closed + recreated.

Checklist

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• [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.
• [x] 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.