Remove unused exports

[Glittersup]

Summary

Some modules export values nothing imports, which makes the public surface of each module look larger than it is and invites accidental dependencies on internals.

Acceptance criteria

• Exports with no importers are made private or removed.
• The typecheck and the suite still pass.
• No behaviour changes.

Getting started

Fork this repository, clone your fork, and add this repo as upstream:

git clone https://github.com/<your-username>/tricklepay-backend.git
cd tricklepay-backend
git remote add upstream https://github.com/TricklePay/tricklepay-backend.git

Create a branch for this issue:

git checkout -b refactor/issue-161

Suggested commit message:

refactor: remove unused exports

Run npm run typecheck, npm test, and npm run build before opening a pull request and linking this issue.