Fix final-review findings for Playwright fallback

- server.js: catch browser-launch failures inside the injected
  fetchAndExtractPriceViaBrowser wrapper so they route through
  checkProduct's normal error classification (recordCheckFailure)
  instead of escaping as a thrown error that bypasses the DB write.
- server.js: add an in-flight guard around runCheck so overlapping
  callers (cron + check-now) share one run instead of each launching
  its own Chromium.
- Dockerfile: copy only package.json/package-lock.json/node_modules
  before installing Chromium so the expensive install layer is keyed
  to dependency changes, not every commit; copy the rest of the app
  afterward.
- Dockerfile: clean up apt package lists after the Playwright install
  step, matching the build stage's existing cleanup.
- README.md: note that exercising the fallback locally (not just
  running the test suite) needs `npx playwright install chromium`.
- README.md: correct the fallback description — domcontentloaded
  navigation reads the page before most client-side JS finishes, so
  the fallback mainly helps with HTTP-client blocking, not JS-rendered
  prices; reworded to drop the inaccurate claim.

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

server.js

@@ -37,7 +37,18 @@ function summarize(results) {
   return parts.length > 0 ? parts.join(' ') : 'geen producten';
 }
 
-async function runCheck() {
+let inFlightCheck = null;
+function runCheck() {
+  if (inFlightCheck) {
+    return inFlightCheck;
+  }
+  inFlightCheck = runCheckOnce().finally(() => {
+    inFlightCheck = null;
+  });
+  return inFlightCheck;
+}
+
+async function runCheckOnce() {
   const products = listProducts(db);
   let browserPromise = null;
   function getBrowser() {
@@ -50,8 +61,12 @@ async function runCheck() {
     const deps = {
       fetchAndExtractPrice,
       fetchAndExtractPriceViaBrowser: async (url) => {
-        const browser = await getBrowser();
-        return fetchAndExtractPriceViaBrowser(browser, url);
+        try {
+          const browser = await getBrowser();
+          return await fetchAndExtractPriceViaBrowser(browser, url);
+        } catch (err) {
+          return { price: null, method: null, error: err };
+        }
       },
       sendMail: (payload) => sendPriceChangeEmail(transport, {
         from: process.env.SMTP_FROM,