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 ${

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

@@ -31,6 +31,7 @@ class ChainSettings(BaseSettings):
 
     p2p_bind_host: str = "0.0.0.0"
     p2p_bind_port: int = 8001
+    p2p_node_id: str = ""
 
     proposer_id: str = ""
     proposer_key: Optional[str] = None