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.

.env.example

@@ -1,4 +1,14 @@
 KEEPA_API_KEY=your_keepa_api_key_here
+SP_API_CLIENT_ID=your_sp_api_client_id
+SP_API_CLIENT_SECRET=your_sp_api_client_secret
+SP_API_REFRESH_TOKEN=your_sp_api_refresh_token
+SP_API_REGION=na
+SP_API_MARKETPLACE_ID=ATVPDKIKX0DER
+SP_API_SELLER_ID=your_seller_id
+SP_API_USE_SANDBOX=false
+AWS_ACCESS_KEY_ID=your_aws_access_key_id
+AWS_SECRET_ACCESS_KEY=your_aws_secret_access_key
+# AWS_SESSION_TOKEN=optional_if_using_sts
 REDIS_URL=redis://localhost:6379
 LLM_URL=http://localhost:1234/v1
 LLM_MODEL=default