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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>