fix: use phone-reachable relay URL in wallet-import QR

The "Import from a patient app → QR" code baked the web app's own
API_BASE_URL (typically http://localhost:4000) into the pairing URI, so a
real phone could never reach the relay and "Scan to connect" silently
stalled. Use the backend's server-resolved, device-reachable relay URL
instead (PUBLIC_RELAY_URL, else the request host), surface it on the
WalletPairing type, and document PUBLIC_RELAY_URL in .env.example.

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

backend/src/env.ts

@@ -21,6 +21,10 @@ const schema = z.object({
   UPLOAD_DIR: z.string().min(1).default("./uploads"),
   BETTER_AUTH_URL: z.string().min(1).default("http://localhost:4000"),
   FRONTEND_URL: z.string().min(1).default("http://localhost:3000"),
+  // Public, device-reachable URL of this backend's wallet relay, baked into the
+  // QR a patient scans. Optional — when unset we derive it from the request host
+  // (so opening the web app over the LAN yields a reachable LAN URL).
+  PUBLIC_RELAY_URL: z.string().optional(),
   PORT: z.coerce.number().int().positive().default(4000),
   NODE_ENV: z
     .enum(["development", "production", "test"])

backend/src/routes/patients-wallet.ts

@@ -2,8 +2,11 @@ import { eq } from "drizzle-orm";
 import { Router } from "express";
 import { z } from "zod";
 
+import type { Request } from "express";
+
 import { db } from "../db/index.js";
 import { organization } from "../db/schema/auth.js";
+import { env } from "../env.js";
 import { HttpError } from "../lib/http-error.js";
 import { patientInputSchema } from "../lib/patient-validation.js";
 import { isReceptionOnly } from "../lib/role-scope.js";
@@ -21,6 +24,16 @@ export const patientsWalletRouter = Router();
 
 patientsWalletRouter.use(requireAuth, requireOrg);
 
+// The device-reachable URL the patient's app should connect to (baked into the
+// QR). Prefer an explicit PUBLIC_RELAY_URL; otherwise derive it from the request
+// host so that opening the web app over the LAN yields a reachable LAN URL.
+function resolveRelayUrl(req: Request): string {
+  if (env.PUBLIC_RELAY_URL) return env.PUBLIC_RELAY_URL;
+  const host = req.get("host");
+  if (host) return `${req.protocol}://${host}`;
+  return env.BETTER_AUTH_URL;
+}
+
 const requestSchema = z.object({
   walletNumber: z.string().trim().min(1),
   mode: z.enum(["permanent", "temporary"]).default("permanent"),
@@ -47,7 +60,7 @@ patientsWalletRouter.post(
         input.mode,
         input.durationHours,
       );
-      res.status(201).json({ ...view, ephemeralPubKey });
+      res.status(201).json({ ...view, ephemeralPubKey, relayUrl: resolveRelayUrl(req) });
     } catch (err) {
       next(err);
     }