refactor: standardize config path and add CLI entry point

- Move EnvironmentFile from /opt/aitbc/.env to /etc/aitbc/blockchain.env in all systemd services
- Add main() entry point function to CLI for package installation compatibility
- Update aitbc-blockchain-node.service, aitbc-blockchain-node-dev.service
- Update aitbc-blockchain-rpc.service
- Update aitbc-blockchain-sync.service, aitbc-blockchain-sync-dev.service

This aligns with Linux filesystem hierarchy standards where /etc/ is the proper

systemd/aitbc-blockchain-rpc.service

@@ -7,7 +7,7 @@ Type=simple
 User=root
 Group=root
 WorkingDirectory=/opt/aitbc/apps/blockchain-node
-EnvironmentFile=/opt/aitbc/.env
+EnvironmentFile=/etc/aitbc/blockchain.env
 Environment=PATH=/usr/bin:/usr/local/bin:/usr/bin:/bin
 Environment=PYTHONPATH=/opt/aitbc/apps/blockchain-node/src:/opt/aitbc/apps/blockchain-node/scripts
 ExecStart=/usr/bin/python3 -m uvicorn aitbc_chain.app:app --host ${rpc_bind_host} --port ${rpc_bind_port}