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/1_quick-start.md

@@ -0,0 +1,54 @@
+# Node Quick Start
+
+**10 minutes** — Install, configure, and sync a blockchain node.
+
+## Prerequisites
+
+| Resource | Minimum |
+|----------|---------|
+| CPU | 4 cores |
+| RAM | 16 GB |
+| Storage | 100 GB SSD |
+| Network | 100 Mbps stable |
+
+## 1. Install
+
+```bash
+cd /home/oib/windsurf/aitbc
+python -m venv .venv && source .venv/bin/activate
+pip install -e .
+```
+
+## 2. Initialize & Configure
+
+```bash
+aitbc-chain init --name my-node --network ait-devnet
+```
+
+Edit `~/.aitbc/chain.yaml`:
+```yaml
+node:
+  name: my-node
+  data_dir: ./data
+rpc:
+  bind_host: 0.0.0.0
+  bind_port: 8080
+p2p:
+  bind_port: 7070
+  bootstrap_nodes:
+    - /dns4/node-1.aitbc.com/tcp/7070/p2p/...
+```
+
+## 3. Start & Verify
+
+```bash
+aitbc-chain start
+aitbc-chain status                    # node info + sync progress
+curl http://localhost:8080/rpc/health # RPC health check
+```
+
+## Next
+
+- [2_configuration.md](./2_configuration.md) — Full config reference
+- [3_operations.md](./3_operations.md) — Day-to-day ops
+- [7_monitoring.md](./7_monitoring.md) — Prometheus + dashboards