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/1_quick-start.md

@@ -0,0 +1,38 @@
+# Client Quick Start
+
+**5 minutes** — Install, configure, submit your first job.
+
+## 1. Install & Configure
+
+```bash
+pip install -e .                                        # from monorepo root
+aitbc config set coordinator_url http://localhost:8000
+export AITBC_API_KEY=your-key
+```
+
+## 2. Create Wallet
+
+```bash
+aitbc wallet create --name my-wallet
+```
+
+Save your seed phrase securely.
+
+## 3. Submit a Job
+
+```bash
+aitbc client submit --prompt "Summarize this document" --input data.txt
+```
+
+## 4. Track & Download
+
+```bash
+aitbc client status --job-id <JOB_ID>
+aitbc client download --job-id <JOB_ID> --output ./results
+```
+
+## Next
+
+- [2_job-submission.md](./2_job-submission.md) — Advanced job options (GPU, priority, batch)
+- [3_job-lifecycle.md](./3_job-lifecycle.md) — Status tracking, results, history
+- [5_pricing-billing.md](./5_pricing-billing.md) — Cost structure