Describe Your Changes

This PR is to support jan x cortex integration where:

• Jan needs to know whether the engine is initialized or not.
• Jan should be able to trigger the engine init via API requests.
• Jan should be able to observe the engine's download progress.
• Change default to main branch for default model file lookup
• Fixed download manager does not yield the latest event

init.cli.usecase is deprecated, there should be no cli use cases

Fixes Issues

• Closes #
• Closes #

Self Checklist

• Added relevant comments, esp in complex areas
• Updated docs (for bug fixes / features)
• Created issues for follow-up changes or refactoring needed