chore: standardize configuration, logging, and error handling across blockchain node and coordinator API

- Add infrastructure.md and workflow files to .gitignore to prevent sensitive info leaks
- Change blockchain node mempool backend default from memory to database for persistence
- Refactor blockchain node logger with StructuredLogFormatter and AuditLogger (consistent with coordinator)
- Add structured logging fields: service, module, function, line number
- Unify coordinator config with Database

docs/4_blockchain/3_operations.md

@@ -0,0 +1,102 @@
+# Node Operations
+Day-to-day operations for blockchain nodes.
+
+## Starting the Node
+
+```bash
+# Start in foreground (for testing)
+aitbc-chain start
+
+# Start as daemon
+aitbc-chain start --daemon
+
+# Start with custom config
+aitbc-chain start --config /path/to/config.yaml
+```
+
+## Stopping the Node
+
+```bash
+# Graceful shutdown
+aitbc-chain stop
+
+# Force stop
+aitbc-chain stop --force
+```
+
+## Node Status
+
+```bash
+aitbc-chain status
+```
+
+Shows:
+- Block height
+- Peers connected
+- Mempool size
+- Last block time
+
+## Checking Sync Status
+
+```bash
+aitbc-chain sync-status
+```
+
+Shows:
+- Current height
+- Target height
+- Sync progress percentage
+- Estimated time to sync
+
+## Managing Peers
+
+### List Peers
+
+```bash
+aitbc-chain peers list
+```
+
+### Add Peer
+
+```bash
+aitbc-chain peers add /dns4/new-node.example.com/tcp/7070/p2p/...
+```
+
+### Remove Peer
+
+```bash
+aitbc-chain peers remove <PEER_ID>
+```
+
+## Backup & Restore
+
+### Backup Data
+
+```bash
+aitbc-chain backup --output /backup/chain-backup.tar.gz
+```
+
+### Restore Data
+
+```bash
+aitbc-chain restore --input /backup/chain-backup.tar.gz
+```
+
+## Log Management
+
+```bash
+# View logs
+aitbc-chain logs --tail 100
+
+# Filter by level
+aitbc-chain logs --level error
+
+# Export logs
+aitbc-chain logs --export /var/log/aitbc-chain.log
+```
+
+## Next
+
+- [Quick Start](./1_quick-start.md) — Get started
+- [Configuration](./2_configuration.md) - Configure your node
+- [Consensus](./4_consensus.md) — Consensus mechanism