chore: rearrange directory structure

[mantariksh]

Problem

Repo structure is inconsistent in a couple of ways:

• Some backend tests are within __tests__ folders, while some are in tests/unit/backend
• src/loaders and src/config are outside src/app, even though they are used purely on the server

Solution

• Move all tests into __tests__ folder so they are as close as possible to the modules being tested.
• Move src/loaders and src/config into src/app and fix all relevant imports and mocks.
• Update the backend structure documented in ARCHITECTURE.md.

[karrui]

thanks for the cleanup.
couldn't comment on the PR since they were just renamed, but should logic.spec.ts and file-validation.spec.ts be in the shared folder tests instead, since the files they are testing are not in /app/utils?

lgtm otherwise.

[mantariksh]

@karrui good catch!

[karrui]

lgtm