feat: Integrate Amazon SP-API for product sellability and pricing

- Added `amazon-sp-api` dependency to package.json.
- Enhanced configuration to include SP-API credentials and settings.
- Implemented SP-API client initialization and error handling in sp-api.ts.
- Developed functions to fetch product sellability and pricing data.
- Updated main processing logic in index.ts to incorporate sellability checks before fetching pricing.
- Modified LLM analysis to account for sellability status and reasons.
- Created a new sp-test.ts script for testing SP-API connectivity and sellability.
- Updated types.ts to define SellabilityInfo and extend SpApiData.
- Enhanced result reporting in writer.ts to include sellability information.

package.json

@@ -10,6 +10,7 @@
     "typescript": "^5"
   },
   "dependencies": {
+    "amazon-sp-api": "^1.2.1",
     "ioredis": "^5.10.1",
     "xlsx": "^0.18.5"
   }