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/2_configuration.md

@@ -0,0 +1,87 @@
+# Blockchain Node Configuration
+Configure your blockchain node for optimal performance.
+
+## Configuration File
+
+Location: `~/.aitbc/chain.yaml`
+
+## Node Configuration
+
+```yaml
+node:
+  name: my-node
+  network: ait-devnet  # or ait-mainnet
+  data_dir: /opt/blockchain-node/data
+  log_level: info
+```
+
+## RPC Configuration
+
+```yaml
+rpc:
+  enabled: true
+  bind_host: 0.0.0.0
+  bind_port: 8080
+  cors_origins:
+    - http://localhost:3000
+    - http://localhost:8000
+  rate_limit: 1000  # requests per minute
+```
+
+## P2P Configuration
+
+```yaml
+p2p:
+  enabled: true
+  bind_host: 0.0.0.0
+  bind_port: 7070
+  bootstrap_nodes:
+    - /dns4/node-1.aitbc.com/tcp/7070/p2p/...
+  max_peers: 50
+  min_peers: 5
+```
+
+## Mempool Configuration
+
+```yaml
+mempool:
+  backend: database  # or memory
+  max_size: 10000
+  min_fee: 0
+  eviction_interval: 60
+```
+
+## Database Configuration
+
+```yaml
+database:
+  adapter: postgresql  # or sqlite
+  url: postgresql://user:pass@localhost/aitbc_chain
+  pool_size: 10
+  max_overflow: 20
+```
+
+## Validator Configuration
+
+```yaml
+validator:
+  enabled: true
+  key: <VALIDATOR_PRIVATE_KEY>
+  block_time: 2  # seconds
+  max_block_size: 1000000  # bytes
+  max_txs_per_block: 500
+```
+
+## Environment Variables
+
+```bash
+export AITBC_CHAIN_DATA_DIR=/opt/blockchain-node/data
+export AITBC_CHAIN_RPC_PORT=8080
+export AITBC_CHAIN_P2P_PORT=7070
+```
+
+## Next
+
+- [Quick Start](./1_quick-start.md) — Get started
+- [Operations](./3_operations.md) — Day-to-day ops
+- [Consensus](./4_consensus.md) — Consensus mechanism