feat(mobile): callSignal subscribe + broadcast helpers

apps/mobile/lib/callSignal.ts

@@ -0,0 +1,37 @@
+import { rtc } from '@chat-app/shared';
+import type { CallSignal } from '@chat-app/shared/rtc';
+import type { AppSupabaseClient } from '@chat-app/shared/supabase';
+
+// Thin wrapper around Supabase realtime broadcast for call signaling.
+// One channel per peer userId. Subscriptions live for the lifetime of
+// the AuthProvider's session; teardown returns a no-arg unsubscribe.
+
+export type SignalListener = (signal: CallSignal) => void;
+
+export function subscribeCallSignals(
+  client: AppSupabaseClient,
+  myUserId: string,
+  onSignal: SignalListener,
+): () => void {
+  const ch = client.channel(rtc.signalTopic(myUserId));
+  ch.on('broadcast', { event: 'signal' }, (msg) => {
+    if (msg.payload && typeof msg.payload === 'object') {
+      onSignal(msg.payload as CallSignal);
+    }
+  });
+  void ch.subscribe();
+  return () => {
+    void client.removeChannel(ch);
+  };
+}
+
+export async function sendCallSignal(
+  client: AppSupabaseClient,
+  toUserId: string,
+  payload: CallSignal,
+): Promise<void> {
+  const ch = client.channel(rtc.signalTopic(toUserId));
+  await ch.subscribe();
+  await ch.send({ type: 'broadcast', event: 'signal', payload });
+  await client.removeChannel(ch);
+}