Extract the stream view mapper into one place

[Glittersup]

Summary

The conversion from a stored stream to the API view shape appears in more than one route. Duplicating it means a new field can be added in one response and forgotten in another.

Acceptance criteria

• The mapping from stored stream to API view lives in one function.
• All routes returning a stream use it.
• The responses are unchanged.

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-158

Suggested commit message:

refactor: extract the stream view mapper

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