Chore: Format backend Python codebase using Black

### Description
The Python backend codebase has some formatting inconsistencies. Running the `black` Python formatter locally on the `backend/` directory reveals that approximately 19 files currently do not conform to standard PEP 8 formatting rules via Black.

Adopting a strict formatting standard keeps the codebase clean, consistent, and makes pull request workflows much easier for reviewers going forward.

### Expected Behavior
- Code in the `backend/` directory should be strictly formatted using the `black` formatter.

### Possible Solution
I would like to submit a PR to run `black` across the `backend/` directory (excluding vendor libraries) to standardize the formatting.
To make reviews easier, I would keep this PR focused solely on automated formatting without any structural logic changes.