Go interfaces make test stubbing easy

What this post added

This post introduces the use of Go interfaces to simplify unit testing for components like image compression workers. By defining a `Job` interface, the `Worker` package can accept any type that implements `Do()` and `Priority()`, allowing for the creation of `DummyJob` implementations for testing purposes. This significantly improves test isolation and reduces the complexity of setting up test environments for components that interact with complex underlying systems.