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/2_clients/5_pricing-billing.md

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+# Pricing & Costs
+Understand the cost structure for using AITBC.
+
+## Cost Structure
+
+### Per-Job Pricing
+
+| Resource | Unit | Price |
+|----------|------|-------|
+| GPU (V100) | per hour | 0.05 AITBC |
+| GPU (A100) | per hour | 0.10 AITBC |
+| GPU (RTX3090) | per hour | 0.03 AITBC |
+| Storage | per GB/day | 0.001 AITBC |
+
+### Priority Pricing
+
+| Priority | Multiplier |
+|----------|------------|
+| Low | 0.8x |
+| Normal | 1.0x |
+| High | 1.5x |
+| Urgent | 2.0x |
+
+## Cost Examples
+
+### Small Job (V100, 1 hour)
+
+```
+Base: 0.05 AITBC
+Normal priority: 1.0x
+Total: 0.05 AITBC
+```
+
+### Large Job (A100, 4 GPUs, 4 hours)
+
+```
+Base: 0.10 AITBC × 4 GPUs × 4 hours = 1.60 AITBC
+High priority: 1.5x
+Total: 2.40 AITBC
+```
+
+## Free Tier
+
+- 10 GPU hours per month
+- 1 GB storage
+- Limited to V100 GPUs
+
+## Enterprise Plans
+
+| Feature | Basic | Pro | Enterprise |
+|---------|-------|-----|------------|
+| GPU hours/month | 100 | 500 | Unlimited |
+| Priority | Normal | High | Urgent |
+| Support | Email | 24/7 Chat | Dedicated |
+| SLA | 99% | 99.9% | 99.99% |
+
+## Next Steps
+
+- [Billing](./5_pricing-billing.md) - Billing and invoices
+- [Wallet](./4_wallet.md) - Managing your wallet
+- [Job Submission](./2_job-submission.md) - Submitting jobs
+
+---
+Manage billing and view invoices.
+
+## View Invoices
+
+```bash
+aitbc billing list
+```
+
+### Filter by Date
+
+```bash
+aitbc billing list --from 2026-01-01 --to 2026-01-31
+```
+
+### Download Invoice
+
+```bash
+aitbc billing download --invoice-id <INV_ID>
+```
+
+## Payment Methods
+
+### Add Payment Method
+
+```bash
+aitbc billing add-card --number 4111111111111111 --expiry 12/26 --cvc 123
+```
+
+### Set Default
+
+```bash
+aitbc billing set-default --card-id <CARD_ID>
+```
+
+## Auto-Pay
+
+```bash
+# Enable auto-pay
+aitbc billing auto-pay enable
+
+# Disable auto-pay
+aitbc billing auto-pay disable
+```
+
+## Billing Alerts
+
+```bash
+# Set spending limit
+aitbc billing alert --limit 100 --email you@example.com
+```
+
+## Next Steps
+
+- [Pricing](./5_pricing-billing.md) - Cost structure
+- [Wallet](./4_wallet.md) - Managing your wallet
+- [Job Submission](./2_job-submission.md) - Submitting jobs