fix(arcade): only notify room creator of join requests

Fixes issue where ALL room members were seeing join request approval
toasts, but only the creator can approve them, leading to confusing
error messages when non-creators clicked approve/deny.

Changes:
- Join request notifications now sent only to room creator's user channel
- Changed from broadcasting to entire room to targeted user notification
- Uses `user:${room.createdBy}` channel instead of `room:${roomId}`

Non-host users will no longer see approval toasts they cannot act on.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

apps/web/src/app/api/arcade/rooms/[roomId]/join-requests/route.ts

@@ -88,11 +88,12 @@ export async function POST(req: NextRequest, context: RouteContext) {
       `[Join Requests] Created request for user ${viewerId} (${displayName}) to join room ${roomId}`
     )
 
-    // Broadcast to all members in the room (particularly the host) via socket
+    // Broadcast to the room host (creator) only via socket
     const io = await getSocketIO()
     if (io) {
       try {
-        io.to(`room:${roomId}`).emit('join-request-submitted', {
+        // Send notification only to the room creator's user channel
+        io.to(`user:${room.createdBy}`).emit('join-request-submitted', {
           roomId,
           request: {
             id: request.id,
@@ -103,7 +104,7 @@ export async function POST(req: NextRequest, context: RouteContext) {
         })
 
         console.log(
-          `[Join Requests] Broadcasted join-request-submitted for user ${viewerId} to room ${roomId}`
+          `[Join Requests] Broadcasted join-request-submitted to room creator ${room.createdBy}`
         )
       } catch (socketError) {
         // Log but don't fail the request if socket broadcast fails