feat: Implement supplier export functionality with workbook generation

- Add `writeSupplierWorkbook` function to create Excel workbooks for supplier analysis results.
- Introduce `SupplierExportSummary` type for summarizing export data.
- Create tests for `writeSupplierWorkbook` to ensure correct sheet creation and data population.
- Implement supplier scoring logic in `supplier-scoring.ts` to evaluate product profitability and demand.
- Add tests for supplier scoring to validate scoring logic and verdict determination.
- Enhance UPC file analysis to integrate supplier scoring and export results to Excel.
- Update database writing logic to accommodate new supplier analysis results.
- Refactor types to include supplier-specific data structures and scoring metrics.
- Ensure proper cleanup of temporary files after tests.

src/server.ts

@@ -1,4 +1,5 @@
 import index from "./web/index.html";
+import path from "node:path";
 import { getDb, initDb } from "./database.ts";
 import {
   fetchKeepaDataBatch,
@@ -52,7 +53,7 @@ type ProductListRecord = {
   fetched_at: string;
 };
 
-const DB_PATH = process.env.RESULTS_DB_PATH || "./results.db";
+const DB_PATH = process.env.RESULTS_DB_PATH || path.join("db", "results.db");
 const DEFAULT_PAGE_SIZE = 25;
 const MAX_PAGE_SIZE = 200;
 const ASIN_PATTERN = /^[A-Z0-9]{10}$/;