feat: route wallet traffic through the Temetro Network relay (v0.7.0)

Devices no longer connect to the backend directly. The /wallet Socket.io
namespace is removed from realtime.ts; a new services/relay-client.ts connects
to the standalone Temetro Network relay's /hub namespace (RELAY_TOKEN-auth),
emitToWallet delegates to its sendToWallet, and device responses + wallet:online
replay are handled there via the same wallet-share/wallet-updates services.

- Add RELAY_URL + RELAY_TOKEN env (env.ts, .env.example, docker-compose.yml);
  the wallet-import QR (resolveRelayUrl) now points at RELAY_URL.
- Add socket.io-client dependency.
- Document the Temetro Network folder/service in root + backend CLAUDE.md.
- Bump root/backend/frontend to 0.7.0; CHANGELOG entry.

The relay service itself lives in github.com/temetro/temetro-network.

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

backend/src/env.ts

@@ -28,6 +28,13 @@ const schema = z.object({
   // Overrides the version reported by GET /api/version. Normally derived from
   // package.json; the release pipeline can pin it explicitly.
   APP_VERSION: z.string().optional(),
+  // Temetro Network relay (github.com/temetro/temetro-network). Both this
+  // backend and patient phones connect to it; it routes the encrypted wallet
+  // messages between them. RELAY_URL is the relay's public URL (also baked into
+  // the QR a patient scans); RELAY_TOKEN is the shared secret this backend
+  // presents on the relay's /hub namespace (must match the relay's RELAY_TOKEN).
+  RELAY_URL: z.string().min(1).default("http://localhost:8080"),
+  RELAY_TOKEN: z.string().default(""),
   // 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).
@@ -80,6 +87,11 @@ if (env.NODE_ENV === "production") {
     );
     process.exit(1);
   }
+  if (!env.RELAY_TOKEN) {
+    console.warn(
+      "⚠️  RELAY_TOKEN is unset in production — the Temetro Network /hub connection is unauthenticated. Set a shared secret: openssl rand -base64 32",
+    );
+  }
 }
 
 export const isProd = env.NODE_ENV === "production";