Add p2p_node_id configuration setting and make node-id argument optional with fallback chain

- Add p2p_node_id field to ChainSettings with empty string default
- Add p2p_node_id to .env.example configuration file
- Change --node-id argument from required to optional with default empty string
- Add node_id resolution with fallback chain: args.node_id -> settings.p2p_node_id -> settings.proposer_id
- Add validation to raise ValueError if no node_id can be resolved
- Update systemd service to use ${

systemd/aitbc-blockchain-p2p.service

@@ -10,7 +10,7 @@ WorkingDirectory=/opt/aitbc/apps/blockchain-node
 Environment=PATH=/usr/bin:/usr/local/bin:/usr/bin:/bin
 Environment=PYTHONPATH=/opt/aitbc/apps/blockchain-node/src:/opt/aitbc/apps/blockchain-node/scripts
 EnvironmentFile=/etc/aitbc/blockchain.env
-ExecStart=/opt/aitbc/venv/bin/python -m aitbc_chain.p2p_network --host ${p2p_bind_host} --port ${p2p_bind_port} --peers ${p2p_peers} --node-id ${proposer_id}
+ExecStart=/opt/aitbc/venv/bin/python -m aitbc_chain.p2p_network --host ${p2p_bind_host} --port ${p2p_bind_port} --peers ${p2p_peers} --node-id ${p2p_node_id}
 Restart=always
 RestartSec=5
 StandardOutput=journal