[BE] Endre navn på externalApi -> externalDataModel

Ref. prat om konsepter og komponenter ble vi enige om termene "intern datamodell" for det som nå heter "datamodell" og "ekstern datamodell" for det som nå heter "eksternt API".

Start by searching the repository for references to externalApi, datamodell and eksternt API; the issue does not identify specific files or tests. Update the relevant terminology consistently to externalDataModel and the agreed internal/external data model terms. Done means no stale externalApi naming remains in the affected code and related references still use consistent terminology.

Written by the indexing model from the issue text.