Show a code on the till so a phone can be pointed at this shop

[sridharkalaibala]

Setting up a staff handset meant typing an address, and the person holding the phone is the one who does not know it. The owner knows the shop code; a waiter five minutes into their first shift does not, and had no way to find out.

So the till shows a code and the phone reads it. GET /pair renders a printable page with a QR for every address a phone could actually reach this machine on. Captain already scans it.

Which address goes on the code

That is the whole substance, and getting it wrong is invisible here: the page renders a perfectly good QR for an address the phone cannot reach, and it surfaces minutes later on somebody else's handset as "cannot find the shop".

• A till opened locally sees localhost:5555 in its own address bar, and a phone pointed at localhost is pointed at itself. So addresses come from the network interfaces, not from the request.
• Wi-Fi first, because a till is often wired and wireless and the phone is on the wireless one.
• A shop reached at a public address gets that instead, since it keeps working away from the shop.
• A machine with no network says so rather than inventing an address.

The decision lives in api/src/utils/pairing.js with no dependencies, so it is testable without express, qrcode or a running server. 8 tests cover it.

Public on purpose

Declared in public-routes.test.js with a reason, which is what that test is for. The code carries an address, not a credential - the same thing the till's own address bar shows anybody standing at it. Requiring a login would mean a handset cannot be paired until somebody signs in on the till, which is backwards: pairing is what happens before anyone can.

Notes

• Uses the qrcode package the API already depends on. No new dependencies.
• docs/API.md, docs/openapi.json and the README endpoint count regenerated, as the docs gate requires.
• Full suite at parity with develop: same failures before and after, plus 8 new passing tests.

[github-actions]

Merged to develop. Anyone can test this - you do not need write access.

Try it at https://develop.posnic.io, or run it yourself:

git fetch origin develop && git checkout develop
npm install && npm --prefix api install
npm run dev   # then http://localhost:3000

When you have tested it, say what you did and what happened, and set
QA passed or QA failed. If you cannot set labels, just comment -
a maintainer will.

Reporting that something is broken is as useful as fixing it. It is
better found here than by a shopkeeper.