Task: Swap FluentValidation for Pure Data Annotations

[zysim]

We probably don't need FluentValidation for this project. Data annotations should handle our use cases already, plus we have to add extra, misleading annotations to handle null request field validation. Specifically; we need to annotate [Required(AllowEmptyStrings = true)] in order to fix what the Swagger doc says, even though we don't actually allow empty strings for that request.

Affected Functionality

• Validation, obviously
• Swagger doc accuracy