Extract ES interface from FastAPI routing definition

[pgulley]

Creates queries.py, which contains a QueryBuilder and an EsClientWrapper, separating concerns more explicitly. Also removes unused significant/rare term query logic.
(Addressing #71)

Questions:

• Are there enough modules here now that we want to package the code up in a subdirectory?
• Can we safely remove all of the config.yml logic? We're no longer using it, in favor of setting things via environment variables.

[rahulbot]

Nice work - I like this restructuring a lot better. I do agree that now that there are multiple modules it'd make sense to have a package they all sit in. I made some suggestions and requests for clarifications or comments in my notes.

[kilemensi]

👍🏽

---

1. I agree we need to choose whether to continue using config.yml or not. I like the idea of config/settings module that abstract pulling things from system env vars (or .env file) in a type-safe manner. This being a FastAPI app, my vote would be to switch to using pydantic settings instead of building one from scratch around getnev.
2. On introducing a subdirectory, yeah, an app or src subdirectory would help but the app itself is also very straightforward so I have no issues with not having one.

[philbudne]

I'm at the polar opposite from Rahul (who had enough context to make meaningful review comments). Most of the actual ES workings are outside my comfort zone!

[pgulley]

1. I agree we need to choose whether to continue using `config.yml` or not. I like the idea of `config`/`settings` module that abstract pulling things from system env vars (or `.env` file) in a type-safe manner. This being a FastAPI app, my vote would be to switch to using [`pydantic settings`](https://fastapi.tiangolo.com/advanced/settings/#settings-in-another-module) instead of building one from scratch around `getnev`.

Quick glance at pydantic_settings and I'm impressed- I'll experiment weaving it in now. It should drop-in no problem to the way we're passing in environment variables now, and it would be a one-line change to go back to using a .env file if needed in the future for some reason.

[pgulley]

Floating back through here to reiterate that pydantic_settings is wonderful- one of those tools that makes me wonder how I got by without it. Thanks for the tip @kilemensi

Addressed the majority of issues, left comments on those I'm not addressing.