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/0_readme.md

@@ -0,0 +1,23 @@
+# Blockchain Node Documentation
+
+Run a blockchain node: validate transactions, produce blocks, maintain the AITBC ledger.
+
+## Reading Order
+
+| # | File | What you learn |
+|---|------|----------------|
+| 1 | [1_quick-start.md](./1_quick-start.md) | Get a node running in 10 minutes |
+| 2 | [2_configuration.md](./2_configuration.md) | Node, RPC, P2P, mempool settings |
+| 3 | [3_operations.md](./3_operations.md) | Start/stop, sync, peers, backups |
+| 4 | [4_consensus.md](./4_consensus.md) | PoA consensus mechanism |
+| 5 | [5_validator.md](./5_validator.md) | Become a validator, duties, rewards |
+| 6 | [6_networking.md](./6_networking.md) | Firewall, NAT, bootstrap nodes |
+| 7 | [7_monitoring.md](./7_monitoring.md) | Prometheus, dashboards, alerts |
+| 8 | [8_troubleshooting.md](./8_troubleshooting.md) | Common issues and fixes |
+| 9 | [9_upgrades.md](./9_upgrades.md) | Version upgrades, rollback |
+| 10 | [10_api-blockchain.md](./10_api-blockchain.md) | RPC and WebSocket API reference |
+
+## Related
+
+- [Installation](../0_getting_started/2_installation.md) — Install all components
+- [CLI Guide](../0_getting_started/3_cli.md) — `aitbc blockchain` commands