feat: fall back to the browser scraper when the primary fetch fails

- Extract fetchAndExtractPriceViaBrowser from deps (optional)
- After primary fetch, if it errors or returns no price and fallback exists, try fallback
- Fallback result replaces primary result for further processing
- Retry-safety logic unchanged: last_price/last_checked_at only updated on success
- All 4 new tests pass, all existing tests unchanged

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

src/checkProducts.js

@@ -1,6 +1,13 @@
 async function checkProduct(product, deps) {
-  const { fetchAndExtractPrice, sendMail, recordCheckSuccess, recordCheckFailure, now } = deps;
-  const result = await fetchAndExtractPrice(product.url);
+  const {
+    fetchAndExtractPrice, fetchAndExtractPriceViaBrowser,
+    sendMail, recordCheckSuccess, recordCheckFailure, now,
+  } = deps;
+  let result = await fetchAndExtractPrice(product.url);
+
+  if ((result.error || result.price == null) && fetchAndExtractPriceViaBrowser) {
+    result = await fetchAndExtractPriceViaBrowser(product.url);
+  }
 
   if (result.error) {
     console.error(`[check] fetch_error "${product.name}" (${product.url}): ${result.error.message}`);