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.

src/types.ts

@@ -42,7 +42,13 @@ export interface KeepaData {
   categoryTree: string[];
 }
 
-export interface SpApiData {
+export type SellabilityInfo = {
+  canSell: boolean | null;
+  sellabilityStatus: "available" | "restricted" | "not_available" | "unknown";
+  sellabilityReason?: string;
+};
+
+export interface SpApiData extends SellabilityInfo {
   fbaFee: number;
   fbmFee: number;
   referralFeePercent: number;