fix: prevent presence race condition during WebRTC handshake

Add /joining, /joined, and /leave endpoints to track user join intent
and trigger presence updates.

Backend:
- Add pending_joins Redis module with 30s TTL
- Add /joining endpoint (before WebRTC handshake)
- Add /joined endpoint (after connection, triggers presence poll)
- Add /leave endpoint (on tab close, triggers presence poll)
- Check for pending joins before deactivating meetings in worker

Frontend:
- Generate unique connectionId per browser tab
- Call /joining before Daily.co join, /joined after connection
- Add beforeunload handler calling /leave via sendBeacon

www/app/lib/apiHooks.ts

@@ -807,6 +807,35 @@ export function useRoomJoinMeeting() {
   );
 }
 
+// Presence race fix endpoints (not yet in OpenAPI spec)
+// These signal join intent to prevent race conditions during WebRTC handshake
+// eslint-disable-next-line @typescript-eslint/no-explicit-any
+export function useMeetingJoining(): any {
+  return ($api as any).useMutation(
+    "post",
+    "/v1/rooms/{room_name}/meetings/{meeting_id}/joining",
+    {},
+  );
+}
+
+// eslint-disable-next-line @typescript-eslint/no-explicit-any
+export function useMeetingJoined(): any {
+  return ($api as any).useMutation(
+    "post",
+    "/v1/rooms/{room_name}/meetings/{meeting_id}/joined",
+    {},
+  );
+}
+
+// eslint-disable-next-line @typescript-eslint/no-explicit-any
+export function useMeetingLeave(): any {
+  return ($api as any).useMutation(
+    "post",
+    "/v1/rooms/{room_name}/meetings/{meeting_id}/leave",
+    {},
+  );
+}
+
 export function useRoomIcsSync() {
   const { setError } = useError();