feat: add multi-chain support to blockchain explorer and improve GPU review handling

- Add multi-chain configuration with devnet, testnet, and mainnet RPC URLs
- Add chain selector dropdown in explorer UI for network switching
- Add chain_id parameter to all API endpoints (chain/head, blocks, transactions, search)
- Add /api/chains endpoint to list supported blockchain networks
- Update blockchain explorer port from 3001 to 8016
- Update devnet RPC port from 8080 to 8026
- Add GPU reviews table

systemd/aitbc-coordinator-api.service

@@ -12,7 +12,7 @@ Environment=PYTHONPATH=/opt/aitbc/apps/coordinator-api/src
 Environment=MINER_API_KEYS=["miner_test_abc123"]
 # Python version validation
 ExecStartPre=/bin/bash -c "python3 --version || (echo 'Python 3.13.5+ required' && exit 1)"
-ExecStart=/opt/aitbc/apps/coordinator-api/.venv/bin/python -m uvicorn app.main:app --host 0.0.0.0 --port 8000
+ExecStart=/opt/aitbc/apps/coordinator-api/.venv/bin/python -m uvicorn main:app --host 0.0.0.0 --port 8000
 ExecReload=/bin/kill -HUP $MAINPID
 Restart=always
 RestartSec=5

systemd/aitbc-web-ui.service

@@ -8,13 +8,13 @@ Wants=aitbc-coordinator-api.service
 Type=simple
 User=aitbc
 Group=aitbc
-WorkingDirectory=/opt/aitbc/apps/explorer-web
-Environment=PATH=/opt/aitbc/apps/coordinator-api/.venv/bin
-Environment=PYTHONPATH=/opt/aitbc/apps/coordinator-api/src
+WorkingDirectory=/opt/aitbc/apps/explorer-web/dist
+Environment=PATH=/usr/bin:/bin
+Environment=PYTHONPATH=/opt/aitbc/apps/explorer-web/dist
 Environment=PORT=8016
 Environment=SERVICE_TYPE=web-ui
 Environment=LOG_LEVEL=INFO
-ExecStart=/opt/aitbc/apps/coordinator-api/.venv/bin/python -m uvicorn app.main:app --host 0.0.0.0 --port 8016
+ExecStart=/usr/bin/python3 -m http.server 8016 --bind 127.0.0.1
 ExecReload=/bin/kill -HUP $MAINPID
 Restart=always
 RestartSec=10