Log the real newsletter-subscribe failure reason server-side

The customer-facing message stays generic on purpose (never leak
Brevo's internal error text), but the real reason was discarded
entirely before this — every failure looked identical from the
outside. Cost real debugging time today tracking down a misconfigured
BREVO_LIST_ID in Coolify (was "2", should have been "5") that made
every single signup attempt fail with the same unhelpful message.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

app/api/newsletter/subscribe/route.ts

@@ -24,6 +24,14 @@ export async function POST(req: Request) {
   const source = body.source && VALID_SOURCES.includes(body.source) ? body.source : "newsletter-page";
   const result = await upsertNewsletterContact(email, source);
   if (!result.ok) {
+    // The customer-facing message stays generic on purpose (never leak
+    // Brevo's internal error text to a customer) — but the real reason
+    // was previously discarded entirely, which cost real debugging time
+    // tracking down a misconfigured BREVO_LIST_ID in Coolify (2026-07-25):
+    // every failure looked identical from the outside, whether it was a
+    // bad env var, a Brevo outage, or something else. Logged here so it's
+    // at least diagnosable from the container's own logs going forward.
+    console.error(`newsletter subscribe failed for source=${source}: ${result.reason}`);
     return NextResponse.json({ ok: false, reason: "Anmeldung ist fehlgeschlagen. Bitte versuche es später erneut." }, { status: 502 });
   }
   return NextResponse.json({ ok: true, alreadySubscribed: result.alreadySubscribed ?? false });