feat: add mempool synchronization background task to P2P network service

- Added mempool_sync_loop background task initialization in start() method
- Registered mempool task in _background_tasks list for lifecycle management
- Enables automatic mempool synchronization across P2P mesh network

apps/blockchain-node/src/aitbc_chain/p2p_network.py

@@ -61,6 +61,10 @@ class P2PNetworkService:
         ping_task = asyncio.create_task(self._ping_peers_loop())
         self._background_tasks.append(ping_task)
         
+        # Start background task to sync mempool
+        mempool_task = asyncio.create_task(self._mempool_sync_loop())
+        self._background_tasks.append(mempool_task)
+
         try:
             await self._stop_event.wait()
         finally: