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/0_getting_started/1_intro.md

@@ -0,0 +1,47 @@
+# What is AITBC?
+
+AITBC is a decentralized GPU computing platform that connects AI workloads with GPU providers through a blockchain-coordinated marketplace.
+
+| Role | What you do |
+|------|-------------|
+| **Client** | Rent GPU power, submit AI/ML jobs, pay with AITBC tokens |
+| **Miner** | Provide GPU resources, process jobs, earn AITBC tokens |
+| **Node Operator** | Run blockchain infrastructure, validate transactions |
+
+## Key Components
+
+| Component | Purpose |
+|-----------|---------|
+| Coordinator API | Job orchestration, miner matching, receipt management |
+| Blockchain Node | PoA consensus, transaction ledger, token transfers |
+| Marketplace Web | GPU offer/bid UI, stats dashboard |
+| Trade Exchange | BTC-to-AITBC trading, QR payments |
+| Wallet | Key management, staking, multi-sig support |
+| CLI | 90+ commands across 12 groups for all roles |
+
+## Quick Start by Role
+
+**Clients** → [2_clients/1_quick-start.md](../2_clients/1_quick-start.md)
+```bash
+pip install -e .
+aitbc config set coordinator_url http://localhost:8000
+aitbc client submit --prompt "What is AI?"
+```
+
+**Miners** → [3_miners/1_quick-start.md](../3_miners/1_quick-start.md)
+```bash
+aitbc miner register --name my-gpu --gpu a100 --count 1
+aitbc miner poll
+```
+
+**Node Operators** → [4_blockchain/1_quick-start.md](../4_blockchain/1_quick-start.md)
+```bash
+aitbc-node init --chain-id ait-devnet
+aitbc-node start
+```
+
+## Next Steps
+
+- [2_installation.md](./2_installation.md) — Install all components
+- [3_cli.md](./3_cli.md) — Full CLI usage guide
+- [../README.md](../README.md) — Documentation navigation