feat: add price-check orchestration with retry-safe error handling

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013gwc3aCiyQxmCWQke8RvEz

src/checkProducts.js

@@ -0,0 +1,48 @@
+async function checkProduct(product, deps) {
+  const { fetchAndExtractPrice, sendMail, recordCheckSuccess, recordCheckFailure, now } = deps;
+  const result = await fetchAndExtractPrice(product.url);
+
+  if (result.error) {
+    recordCheckFailure(product.id, 'fetch_error');
+    return { status: 'fetch_error' };
+  }
+  if (result.price == null) {
+    recordCheckFailure(product.id, 'not_found');
+    return { status: 'not_found' };
+  }
+
+  const isBaseline = product.last_price == null;
+  const changed = !isBaseline && result.price !== product.last_price;
+
+  if (changed) {
+    try {
+      await sendMail({
+        name: product.name,
+        url: product.url,
+        oldPrice: product.last_price,
+        newPrice: result.price,
+      });
+    } catch {
+      recordCheckFailure(product.id, 'mail_error');
+      return { status: 'mail_error' };
+    }
+  }
+
+  recordCheckSuccess(product.id, { price: result.price, checkedAt: now() });
+  return { status: 'ok', changed };
+}
+
+async function checkAllProducts(products, deps) {
+  const results = [];
+  for (const product of products) {
+    try {
+      const result = await checkProduct(product, deps);
+      results.push({ productId: product.id, ...result });
+    } catch (err) {
+      results.push({ productId: product.id, status: 'error', error: err });
+    }
+  }
+  return results;
+}
+
+module.exports = { checkProduct, checkAllProducts };