Switch newsletter signup to Brevo double opt-in

Was a plain POST /v3/contacts upsert (single opt-in — straight onto the
list, no confirmation required). Now calls doubleOptinConfirmation
instead, so a signup only requests subscription; Brevo sends its own
confirmation email and adds the contact to the real list only once they
click through. Needs BREVO_DOUBLE_OPTIN_TEMPLATE_ID set in Coolify
before this works — not yet configured, signups will fail closed with a
logged reason until it is.

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

app/lib/brevo.ts

@@ -1,10 +1,20 @@
-// Server-only — syncs newsletter opt-ins to Brevo's Contacts API. Brevo
-// owns everything downstream of that (list membership, unsubscribe links,
-// and whatever Welcome Flow automation is configured on the list in
-// Brevo's own UI — that automation isn't manageable via their public API
-// at all, only contacts/lists are). This app never sends marketing mail
-// itself; it only ever hands Brevo the contact + consent.
-const BREVO_API_URL = "https://api.brevo.com/v3/contacts";
+// Server-only — syncs newsletter opt-ins to Brevo's Contacts API via the
+// double-opt-in endpoint: this only ever *requests* a subscription, it
+// does not add the contact to the real list itself — Brevo sends the
+// confirmation email (the template at BREVO_DOUBLE_OPTIN_TEMPLATE_ID,
+// configured as this list's Double Opt-in template in Brevo's own UI) and
+// only adds the contact to BREVO_LIST_ID once they click through. This
+// app never sends marketing mail itself, and — as of this switch — never
+// even directly grants list membership; it only ever hands Brevo the
+// contact + consent-to-be-asked. Everything after that (the confirmation
+// email itself, the post-confirmation Welcome Flow automation) is
+// configured in Brevo's own UI, not manageable via their public API.
+//
+// Previously called the plain `POST /v3/contacts` upsert (single
+// opt-in — added straight to the list, no confirmation click required).
+// Switched 2026-07-25 per explicit request once the confirmation-email
+// template existed to point templateId at.
+const BREVO_DOUBLE_OPTIN_URL = "https://api.brevo.com/v3/contacts/doubleOptinConfirmation";
 
 export type BrevoSyncResult = { ok: true } | { ok: false; reason: string };
 
@@ -19,12 +29,17 @@ export async function upsertNewsletterContact(
 ): Promise<BrevoSyncResult> {
   const apiKey = process.env.BREVO_API_KEY;
   const listId = process.env.BREVO_LIST_ID;
-  if (!apiKey || !listId) {
-    return { ok: false, reason: "BREVO_API_KEY/BREVO_LIST_ID nicht konfiguriert." };
+  const templateId = process.env.BREVO_DOUBLE_OPTIN_TEMPLATE_ID;
+  if (!apiKey || !listId || !templateId) {
+    return { ok: false, reason: "BREVO_API_KEY/BREVO_LIST_ID/BREVO_DOUBLE_OPTIN_TEMPLATE_ID nicht konfiguriert." };
   }
+  // No dedicated "danke, bestätigt" landing page exists yet — falls back
+  // to the homepage rather than blocking double opt-in on that page
+  // existing first. Revisit once one exists.
+  const redirectionUrl = process.env.BREVO_DOI_REDIRECT_URL || "https://einfach-produktiv.mk360.de/";
 
   try {
-    const res = await fetch(BREVO_API_URL, {
+    const res = await fetch(BREVO_DOUBLE_OPTIN_URL, {
       method: "POST",
       headers: {
         "Content-Type": "application/json",
@@ -32,16 +47,21 @@ export async function upsertNewsletterContact(
       },
       body: JSON.stringify({
         email,
-        listIds: [Number(listId)],
-        updateEnabled: true,
+        includeListIds: [Number(listId)],
+        templateId: Number(templateId),
+        redirectionUrl,
         attributes: { OPT_IN_SOURCE: source },
       }),
       signal: AbortSignal.timeout(8000),
     });
-    // 204 for both a fresh contact and an existing one (updateEnabled
-    // above merges the list membership onto the existing contact instead
-    // of erroring).
-    if (res.ok || res.status === 204) return { ok: true };
+    // 201 Created is this endpoint's success status (unlike the plain
+    // contacts upsert this replaced, which used 204). A contact who's
+    // already confirmed-and-subscribed re-submitting the form is not
+    // treated as an error either — Brevo resends the confirmation email
+    // in that case rather than erroring, which is an acceptable no-op
+    // resend from this app's point of view (matches the previous
+    // endpoint's "always succeeds for an existing contact too" behavior).
+    if (res.ok || res.status === 201) return { ok: true };
     const body = await res.json().catch(() => null);
     return { ok: false, reason: body?.message ?? `Brevo antwortete mit ${res.status}` };
   } catch (err) {