chore: Retrieve `engine` from DB instead of `model.yml`

[gau-nernst]

Problem Statement

A lot of code does the following

• Get model entry from DB
• Get path to model.yml. Read model.yml to obtain engine value

e.g.
https://github.com/menloresearch/cortex.cpp/blob/b31878db2580056232b9841a6e0b42e63cca8167/engine/controllers/models.cc#L292-L297
https://github.com/menloresearch/cortex.cpp/blob/b31878db2580056232b9841a6e0b42e63cca8167/engine/services/model_service.cc#L495-L499

This is unnecessary because the DB already has column engine

We must make sure the column engine is registered correctly e.g. Currently downloading single GGUF file does not fill in column engine

https://github.com/menloresearch/cortex.cpp/blob/b31878db2580056232b9841a6e0b42e63cca8167/engine/services/model_service.cc#L75-L85

This is also a prerequisite for #2151

Feature Idea

See above

[github-roushan]

@gau-nernst.
can i take this one