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

test/checkProducts.test.js

@@ -119,3 +119,66 @@ test('checkAllProducts processes every product independently, isolating failures
   assert.equal(results[0].status, 'error');
   assert.equal(results[1].status, 'ok');
 });
+
+test('checkProduct tries the browser fallback when the primary fetch errors, and uses its result', async (t) => {
+  captureErrorLog(t);
+  const browserCalls = [];
+  const { deps, successes, mailCalls } = makeDeps({
+    fetchAndExtractPrice: async () => ({ price: null, method: null, error: new Error('down') }),
+    fetchAndExtractPriceViaBrowser: async (url) => {
+      browserCalls.push(url);
+      return { price: 10, method: 'json-ld', error: null };
+    },
+  });
+  const product = { id: 7, name: 'X', url: 'https://x', last_price: null };
+  const result = await checkProduct(product, deps);
+  assert.equal(result.status, 'ok');
+  assert.deepEqual(browserCalls, ['https://x']);
+  assert.equal(successes.length, 1);
+  assert.equal(mailCalls.length, 0);
+});
+
+test('checkProduct tries the browser fallback when the primary fetch finds no price', async (t) => {
+  captureErrorLog(t);
+  const browserCalls = [];
+  const { deps, successes } = makeDeps({
+    fetchAndExtractPrice: async () => ({ price: null, method: null, error: null }),
+    fetchAndExtractPriceViaBrowser: async (url) => {
+      browserCalls.push(url);
+      return { price: 12, method: 'regex', error: null };
+    },
+  });
+  const product = { id: 8, name: 'X', url: 'https://x', last_price: null };
+  const result = await checkProduct(product, deps);
+  assert.equal(result.status, 'ok');
+  assert.deepEqual(browserCalls, ['https://x']);
+  assert.equal(successes.length, 1);
+});
+
+test('checkProduct never calls the browser fallback when the primary fetch already succeeds', async () => {
+  const browserCalls = [];
+  const { deps } = makeDeps({
+    fetchAndExtractPriceViaBrowser: async (url) => {
+      browserCalls.push(url);
+      return { price: 999, method: 'meta', error: null };
+    },
+  });
+  const product = { id: 9, name: 'X', url: 'https://x', last_price: null };
+  await checkProduct(product, deps);
+  assert.equal(browserCalls.length, 0);
+});
+
+test('checkProduct records fetch_error when both the primary fetch and the browser fallback fail', async (t) => {
+  const logged = captureErrorLog(t);
+  const { deps, failures, successes } = makeDeps({
+    fetchAndExtractPrice: async () => ({ price: null, method: null, error: new Error('down') }),
+    fetchAndExtractPriceViaBrowser: async () => ({ price: null, method: null, error: new Error('still down') }),
+  });
+  const product = { id: 10, name: 'X', url: 'https://x', last_price: 10 };
+  const result = await checkProduct(product, deps);
+  assert.equal(result.status, 'fetch_error');
+  assert.deepEqual(failures, [{ id: 10, status: 'fetch_error' }]);
+  assert.equal(successes.length, 0);
+  assert.equal(logged.length, 1);
+  assert.match(logged[0], /fetch_error.*still down/);
+});