fix(auth): logar o motivo da recusa do Resend e a causa de fetch failed

src/server/auth/auth.ts

@@ -75,7 +75,7 @@ export function createAuth({
           // Not awaited on purpose (timing attacks), but kept alive with after(): on Vercel the function can be frozen
           // as soon as the response is sent, which silently drops a fire-and-forget fetch. Errors never include the body.
           const task = send({ to: email, subject: "Seu código de acesso ao Compasso", text: `Seu código: ${otp}\nVálido por 5 minutos.` }).catch(
-            (e: unknown) => console.error("[auth] failed to send sign-in code", e instanceof Error ? e.message : e),
+            (e: unknown) => console.error("[auth] failed to send sign-in code", e instanceof Error ? `${e.message}${e.cause ? ` (cause: ${String(e.cause)})` : ""}` : e),
           );
           try {
             after(task);

src/server/auth/mail.ts

@@ -19,5 +19,9 @@ export const sendMail: Mailer = async (message) => {
     headers: { Authorization: `Bearer ${env.RESEND_API_KEY}`, "Content-Type": "application/json" },
     body: JSON.stringify({ from: env.MAIL_FROM, ...message }),
   });
-  if (!res.ok) throw new Error(`Resend responded ${res.status}`);
+  if (!res.ok) {
+    // Resend's error body is {name, message} (e.g. "Invalid `from` field"): useful and free of secrets/OTPs.
+    const detail = await res.json().then((b: { name?: string; message?: string }) => `${b.name ?? ""}: ${b.message ?? ""}`).catch(() => "");
+    throw new Error(`Resend responded ${res.status} ${detail}`.trim());
+  }
 };