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/5_reference/0_index.md

@@ -0,0 +1,28 @@
+# Technical Reference
+
+Specifications, audits, and implementation records for AITBC internals.
+
+## Reading Order
+
+| # | File | Topic |
+|---|------|-------|
+| 1 | [1_cli-reference.md](./1_cli-reference.md) | Full CLI command reference (90+ commands) |
+| 2 | [2_payment-architecture.md](./2_payment-architecture.md) | Payment system design |
+| 3 | [3_wallet-coordinator-integration.md](./3_wallet-coordinator-integration.md) | Wallet ↔ coordinator flow |
+| 4 | [4_confidential-transactions.md](./4_confidential-transactions.md) | Confidential tx architecture + implementation |
+| 5 | [5_zk-proofs.md](./5_zk-proofs.md) | ZK receipt attestation design + comparison |
+| 6 | [6_enterprise-sla.md](./6_enterprise-sla.md) | Enterprise SLA definitions |
+| 7 | [7_threat-modeling.md](./7_threat-modeling.md) | Privacy feature threat model |
+| 8 | [8_blockchain-deployment-summary.md](./8_blockchain-deployment-summary.md) | Node deployment record |
+| 9 | [9_payment-integration-complete.md](./9_payment-integration-complete.md) | Payment integration status |
+| 10 | [10_implementation-complete-summary.md](./10_implementation-complete-summary.md) | Feature completion record |
+| 11–14 | `11_`–`14_` | Integration test fixes, updates, status reports |
+| 15 | [15_skipped-tests-roadmap.md](./15_skipped-tests-roadmap.md) | Skipped tests plan |
+| 16 | [16_security-audit-2026-02-13.md](./16_security-audit-2026-02-13.md) | Security audit results |
+| 17 | [17_docs-gaps.md](./17_docs-gaps.md) | Documentation gap analysis |
+
+## Related
+
+- [Architecture](../6_architecture/) — System design docs
+- [Security](../9_security/) — Security guides
+- [Roadmap](../1_project/2_roadmap.md) — Development roadmap