Previous SQLite-related code used context.Background() and context.TODO() because it was not developed with context awareness.
This commit propagates the main Steve context so that it can be used when interacting with SQL context-aware functions.
This PR removes all production-code use of context.Background() and context.TODO() and replaces test-code use of TODO with Background.
Contributes to rancher/rancher#47825
This introduces the a `WithTransaction` function, which is then used for all transactional work in Steve.
Because `WithTransaction` takes care of all `Begin`s, `Commit`s and `Rollback`s, it eliminates the problem where forgotten open transactions can block all other operations (with long stalling and `SQLITE_BUSY` errors).
This also:
- merges together the disparate `DBClient` interfaces in one only `db.Client` interface with one unexported non-test implementation. I found this much easier to follow
- refactors the transaction package in order to make it as minimal as possible, and as close to the wrapped `sql.Tx` and `sql.Stmt` functions as possible, in order to reduce cognitive load when working with this part of the codebase
- simplifies tests accordingly
- adds a couple of known files to `.gitignore`
Credits to @tomleb for suggesting the approach: https://github.com/rancher/lasso/pull/121#pullrequestreview-2515872507
