Which FHIR backend should Last EHR support next?

[cbetz]

Medplum is the supported authenticated backend today, and the bundled HAPI stack covers local synthetic evaluation. The adapter seam is ready for a second authenticated backend: there is a documented contract (docs/adapters.md), an executable starter (examples/fhir-adapter-starter), and shared contract tests.

Open spikes: #73 (Aidbox), #74 (Firely Server), #75 (Oystehr).

What I want to learn from you:

1. Which backend would you actually integrate against in the next few months?
2. What auth mode does your deployment use (client credentials, SMART, static token)?
3. Would you be willing to help validate an adapter against a synthetic project on your stack?

The next adapter ships when someone with a real deployment will co-validate it, so concrete answers here directly set the order of work.