Prevent stale cached prices after barcode selection

[sridharkalaibala]

A fresh barcode lookup could display the current price while selecting that product reused an older detail-cache entry for up to 60 seconds. Discard the selected ID's cached details before adding it, so the basket fetches current product data.

A synthetic regression reproduces a £15 chooser result adding a cached £1.20 price before the fix and verifies the correct ID and fresh price afterward. This establishes a generic defect, not the cause of a particular historical transaction.

Validation: 24 related tests pass; syntax, diff whitespace and attribution checks pass.

[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.