feat: read-only FHIR R4 server (share records over /fhir)

Expose temetro's own records as a read-only FHIR R4 server at /fhir,
authenticated with per-clinic API keys (tmf_… bearer tokens, SHA-256
hashed, shown once). Serves Patient, Observation (labs + vitals),
AllergyIntolerance, Condition, MedicationRequest, Encounter and
Appointment as text-only CodeableConcepts (temetro stores free-text
clinical values); CapabilityStatement at /fhir/metadata (unauth).
Searchset Bundles with _count/_offset pagination and self/next/prev
links; every request is org-scoped and written to the activity log.
Keys are created/revoked under Settings → Integrations (owner/admin).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/src/middleware/fhir-auth.ts

@@ -0,0 +1,49 @@
+import type { NextFunction, Request, Response } from "express";
+
+import { resolveKey } from "../services/fhir-server/keys.js";
+import {
+  FHIR_CONTENT_TYPE,
+  operationOutcome,
+} from "../services/fhir-server/outcome.js";
+
+// Bearer-token auth for the read-only FHIR server. Unlike the rest of the API
+// (Better Auth session cookies), the `/fhir` endpoints authenticate with a
+// per-clinic API key: `Authorization: Bearer tmf_<secret>`. On success the
+// caller's organization is attached to `req.organizationId` and every downstream
+// query is scoped to it. Failures return a FHIR OperationOutcome, not our
+// standard error JSON.
+export async function requireFhirKey(
+  req: Request,
+  res: Response,
+  next: NextFunction,
+): Promise<void> {
+  const header = req.headers.authorization ?? "";
+  const match = /^Bearer\s+(.+)$/i.exec(header.trim());
+  const secret = match?.[1]?.trim();
+
+  const unauthorized = (diagnostics: string) => {
+    res
+      .status(401)
+      .type(FHIR_CONTENT_TYPE)
+      .set("WWW-Authenticate", "Bearer")
+      .json(operationOutcome("error", "login", diagnostics));
+  };
+
+  if (!secret) {
+    unauthorized("Missing bearer token. Send Authorization: Bearer tmf_…");
+    return;
+  }
+
+  try {
+    const resolved = await resolveKey(secret);
+    if (!resolved) {
+      unauthorized("Invalid or revoked API key.");
+      return;
+    }
+    req.organizationId = resolved.orgId;
+    req.fhirKey = { id: resolved.keyId, name: resolved.keyName };
+    next();
+  } catch (err) {
+    next(err);
+  }
+}