feat: log failures, validate env at startup, load .env (findings 3, 4, 7)

Finding 3: fetch_error, not_found, mail_error and de onverwachte-fout
branch loggen nu naam/url plus reden naar stderr; runCheck logt na elke
run een regel met de tellingen per status, zodat een cron-run zichtbaar
is in de container-logs. De recordCheckSuccess/recordCheckFailure-condities
en alle return-waarden zijn ongewijzigd.

Finding 4: server.js controleert bij het opstarten dat BASIC_AUTH_USER,
BASIC_AUTH_PASS, SMTP_FROM en NOTIFY_EMAIL gezet zijn en stopt anders met
exit code 1 in plaats van later opake 500's te geven.

Finding 7: dotenv toegevoegd en als eerste regel in server.js geladen,
zodat het gedocumenteerde `cp .env.example .env && npm start` echt werkt.

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

test/checkProducts.test.js

@@ -17,6 +17,14 @@ function makeDeps(overrides = {}) {
   return { deps, failures, successes, mailCalls };
 }
 
+// Failure paths log to stderr; capture it so the test output stays clean
+// while still proving the logging happened.
+function captureErrorLog(t) {
+  const logged = [];
+  t.mock.method(console, 'error', (...args) => { logged.push(args.join(' ')); });
+  return logged;
+}
+
 test('checkProduct stores baseline price without sending mail', async () => {
   const { deps, successes, mailCalls } = makeDeps();
   const product = { id: 1, name: 'X', url: 'https://x', last_price: null };
@@ -48,7 +56,8 @@ test('checkProduct does not send mail or update when price unchanged', async ()
   assert.equal(successes.length, 1);
 });
 
-test('checkProduct records fetch_error and does not touch price on network failure', async () => {
+test('checkProduct records fetch_error and does not touch price on network failure', async (t) => {
+  const logged = captureErrorLog(t);
   const { deps, failures, successes } = makeDeps({
     fetchAndExtractPrice: async () => ({ price: null, method: null, error: new Error('down') }),
   });
@@ -57,9 +66,12 @@ test('checkProduct records fetch_error and does not touch price on network failu
   assert.equal(result.status, 'fetch_error');
   assert.deepEqual(failures, [{ id: 4, status: 'fetch_error' }]);
   assert.equal(successes.length, 0);
+  assert.equal(logged.length, 1);
+  assert.match(logged[0], /fetch_error.*https:\/\/x.*down/);
 });
 
-test('checkProduct records not_found when no price could be extracted', async () => {
+test('checkProduct records not_found when no price could be extracted', async (t) => {
+  const logged = captureErrorLog(t);
   const { deps, failures, successes } = makeDeps({
     fetchAndExtractPrice: async () => ({ price: null, method: null, error: null }),
   });
@@ -68,9 +80,12 @@ test('checkProduct records not_found when no price could be extracted', async ()
   assert.equal(result.status, 'not_found');
   assert.deepEqual(failures, [{ id: 5, status: 'not_found' }]);
   assert.equal(successes.length, 0);
+  assert.equal(logged.length, 1);
+  assert.match(logged[0], /not_found.*https:\/\/x/);
 });
 
-test('checkProduct records mail_error and keeps old price when sending mail fails', async () => {
+test('checkProduct records mail_error and keeps old price when sending mail fails', async (t) => {
+  const logged = captureErrorLog(t);
   const { deps, failures, successes } = makeDeps({
     sendMail: async () => { throw new Error('smtp down'); },
   });
@@ -79,9 +94,12 @@ test('checkProduct records mail_error and keeps old price when sending mail fail
   assert.equal(result.status, 'mail_error');
   assert.deepEqual(failures, [{ id: 6, status: 'mail_error' }]);
   assert.equal(successes.length, 0);
+  assert.equal(logged.length, 1);
+  assert.match(logged[0], /mail_error.*https:\/\/x.*smtp down/);
 });
 
-test('checkAllProducts processes every product independently, isolating failures', async () => {
+test('checkAllProducts processes every product independently, isolating failures', async (t) => {
+  captureErrorLog(t);
   const { deps } = makeDeps();
   let calls = 0;
   const throwingDeps = {