Change blockchain node and RPC services to use venv Python interpreter and update node service entry point

- Update aitbc-blockchain-node.service ExecStart from blockchain_simple.py to aitbc_chain.combined_main module
- Update aitbc-blockchain-rpc.service ExecStart from /usr/bin/python3 to /opt/aitbc/venv/bin/python
- Ensures both services use virtual environment Python interpreter for consistency

systemd/aitbc-blockchain-node.service

@@ -14,7 +14,7 @@ Environment=PYTHONPATH=/opt/aitbc/services
 EnvironmentFile=/etc/aitbc/production.env
 
 # Production execution
-ExecStart=/opt/aitbc/venv/bin/python /opt/aitbc/services/blockchain_simple.py
+ExecStart=/opt/aitbc/venv/bin/python -m aitbc_chain.combined_main
 ExecReload=/bin/kill -HUP $MAINPID
 KillMode=mixed
 TimeoutStopSec=10

systemd/aitbc-blockchain-rpc.service

@@ -10,7 +10,7 @@ WorkingDirectory=/opt/aitbc/apps/blockchain-node
 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}
+ExecStart=/opt/aitbc/venv/bin/python -m uvicorn aitbc_chain.app:app --host ${rpc_bind_host} --port ${rpc_bind_port}
 Restart=always
 RestartSec=5
 StandardOutput=journal