Transform threads into tasks

MongoDB beanie client doesn't seem to work well with multi-threaded application due to the way how ORM works and how it maps the actual Python classes to MongoDB collections. For now we have somewhat salvaged the main issue regarding this by enabling `multiprocessing_mode` for beanie, but it doesn't make the application thread-safe.

Ideally, in order for us to work with MongoDB application in a thread-safe manner, we would want to create a separate client within each thread, which we could and tried to do actually, but ORM logic overwrites the Python class definitions associated with document model that are shared between threads, which then leads to errors pertaining to event loops (simply put, the Python classes are always rewritten with the latest MongoDB client tied to the newly created thread/job, and then those Python classes are unusable for ORM purporses in other threads...)

We need to convert the following threads into tasks of the primary FastAPI thread:
- Embedding thread
- Search thread
- Setup job
- Recurring jobs (we need to change executor to create a task rather than a thread for recurring jobs)

Keep in mind that the initial reason for utilizing threads in the first place was the blocking nature of various AI inference computations which needs to be now explicitly separated into their own temporary threads in order not to block the execution of the main FastAPI thread. And obviously, we cannot put any code that interacts with MongoDB database into the auxiliary thread since that was the main reason why we need to convert these currently existing threads in the first place.