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/4_consensus.md

@@ -0,0 +1,65 @@
+# Consensus Mechanism
+Understand AITBC's proof-of-authority consensus mechanism.
+
+## Overview
+
+AITBC uses a Proof-of-Authority (PoA) consensus mechanism with:
+- Fixed block time: 2 seconds
+- Authority set of validated proposers
+- Transaction finality on each block
+
+## Block Production
+
+### Proposer Selection
+
+Proposers take turns producing blocks in a round-robin fashion. Each proposer gets a fixed time slot.
+
+### Block Structure
+
+```json
+{
+  "header": {
+    "height": 100,
+    "timestamp": "2026-02-13T10:00:00Z",
+    "proposer": "ait-devnet-proposer-1",
+    "parent_hash": "0xabc123...",
+    "state_root": "0xdef456...",
+    "tx_root": "0xghi789..."
+  },
+  "transactions": [...],
+  "receipts": [...]
+}
+```
+
+## Consensus Rules
+
+1. **Block Time**: 2 seconds minimum
+2. **Block Size**: 1 MB maximum
+3. **Transactions**: 500 maximum per block
+4. **Fee**: Minimum 0 (configurable)
+
+## Validator Requirements
+
+| Requirement | Value |
+|-------------|-------|
+| Uptime | 99% minimum |
+| Latency | < 100ms to peers |
+| Stake | 1000 AITBC |
+
+## Fork Selection
+
+Longest chain rule applies:
+- Validators always extend the longest known chain
+- Reorgs occur only on conflicting blocks within the last 10 blocks
+
+## Finality
+
+Blocks are considered final after:
+- 1 confirmation for normal transactions
+- 3 confirmations for high-value transactions
+
+## Next
+
+- [Quick Start](./1_quick-start.md) — Get started
+- [Validator Operations](./5_validator.md) - Validator guide
+- [Networking](./6_networking.md) - P2P networking