feat: settings and deploy docs

app/api/settings/test-mail/route.ts

@@ -0,0 +1,14 @@
+import { NextRequest, NextResponse } from "next/server";
+import { sendMail } from "@/lib/mail";
+
+export async function POST(req: NextRequest) {
+  const body = await req.json().catch(() => ({}));
+  const to = String(body.to ?? process.env.MAIL_FROM ?? "");
+  if (!to) return NextResponse.json({ error: "Empfänger fehlt" }, { status: 400 });
+  try {
+    await sendMail(to, "Gutschein-Tracker Test-Mail", "Test-Mail vom Gutschein-Tracker: Versand funktioniert.");
+    return NextResponse.json({ ok: true });
+  } catch (e) {
+    return NextResponse.json({ error: e instanceof Error ? e.message : String(e) }, { status: 500 });
+  }
+}