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/6_api-reference.md

@@ -0,0 +1,124 @@
+# Client API Reference
+
+REST API endpoints for client operations.
+
+## Endpoints
+
+### Submit Job
+
+```
+POST /v1/jobs
+```
+
+**Request Body:**
+
+```json
+{
+  "model": "gpt2",
+  "input": "string or file_id",
+  "output_config": {
+    "destination": "local or s3://bucket/path",
+    "format": "json"
+  },
+  "requirements": {
+    "gpu_type": "v100",
+    "gpu_count": 1,
+    "min_vram_gb": 16
+  },
+  "priority": "normal",
+  "timeout_seconds": 3600
+}
+```
+
+**Response:**
+
+```json
+{
+  "job_id": "job_abc123",
+  "estimated_cost": 0.05,
+  "estimated_time_seconds": 600
+}
+```
+
+### Get Job Status
+
+```
+GET /v1/jobs/{job_id}
+```
+
+**Response:**
+
+```json
+{
+  "job_id": "job_abc123",
+  "status": "running",
+  "progress": 45,
+  "miner_id": "miner_xyz789",
+  "created_at": "2026-02-13T10:00:00Z",
+  "started_at": "2026-02-13T10:01:00Z",
+  "completed_at": null,
+  "result": null
+}
+```
+
+### List Jobs
+
+```
+GET /v1/jobs?status=running&limit=10
+```
+
+**Response:**
+
+```json
+{
+  "jobs": [
+    {
+      "job_id": "job_abc123",
+      "status": "running",
+      "model": "gpt2"
+    }
+  ],
+  "total": 1,
+  "has_more": false
+}
+```
+
+### Cancel Job
+
+```
+DELETE /v1/jobs/{job_id}
+```
+
+### Download Results
+
+```
+GET /v1/jobs/{job_id}/download
+```
+
+### Get Job History
+
+```
+GET /v1/jobs/history?from=2026-01-01&to=2026-01-31
+```
+
+## Error Codes
+
+| Code | Description |
+|------|-------------|
+| 400 | Invalid request |
+| 401 | Unauthorized |
+| 404 | Job not found |
+| 422 | Validation error |
+| 429 | Rate limited |
+| 500 | Server error |
+
+## Rate Limits
+
+- 60 requests/minute
+- 1000 requests/hour
+
+## Next
+
+- [1_quick-start.md](./1_quick-start.md) — Get started quickly
+- [2_job-submission.md](./2_job-submission.md) — CLI-based job submission
+- [CLI Guide](../0_getting_started/3_cli.md) — Full CLI reference