fix: default RELAY_URL to the hosted relay (v0.8.2)

The default http://localhost:8080 silently failed for clinics that joined
the network without setting RELAY_URL — inside Docker localhost is the
container itself, so the hub connection never reached the relay (endless
"relay unreachable" retries) and pairing QRs encoded an unreachable
localhost. Default to https://network.temetro.com so "Join Temetro Network"
works out of the box; self-hosters running their own relay still override
it. Also (re)ensure the hub is connected before pre-registering a pairing.

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

backend/src/services/relay-client.ts

@@ -49,7 +49,9 @@ export function expectResponse(orgId: string, requestId: string): void {
 }
 
 // Open (and authenticate) a hub connection for a clinic, if not already open.
-// Idempotent — safe to call on startup and again when an org joins the network.
+// Idempotent — safe to call on startup, when an org joins the network, and
+// before generating a pairing QR. The socket auto-reconnects on its own, so an
+// existing entry is left as-is.
 export async function connectOrg(orgId: string): Promise<void> {
   if (hubs.has(orgId)) return;