feat(admin): CSV-Export der Kunden für die Buchhaltung

GET /admin/export/accounts.csv — eine Zeile pro Kunde (Profil + aktuelles
Abo + Plan-Preis + Instanzen). Semikolon-getrennt, UTF-8 BOM (Excel-CH),
Content-Disposition mit Datum. Nur mit Admin-Token (401 sonst).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

server/routes/admin.js

@@ -99,6 +99,45 @@ adminRouter.get("/accounts/:id", async (req, res) => {
   res.json({ account, subscriptions, instances });
 });
 
+// — CSV-Export für die Buchhaltung —
+// Eine Zeile pro Kunde mit aktuellem Abo + Plan-Preis. Semikolon-getrennt
+// (Excel-CH-freundlich), UTF-8 BOM, damit Umlaute korrekt erscheinen.
+function csvCell(v) {
+  const s = v == null ? "" : String(v);
+  return /[";\n]/.test(s) ? '"' + s.replace(/"/g, '""') + '"' : s;
+}
+adminRouter.get("/export/accounts.csv", async (_req, res) => {
+  const { rows } = await query(`
+    select a.email, a.company, a.contact_name, a.street, a.zip, a.city, a.country, a.created_at,
+           s.plan, s.status as sub_status, s.current_period_end,
+           coalesce(i.cnt, 0)::int as instance_count
+      from accounts a
+      left join lateral (
+        select plan, status, current_period_end from subscriptions
+        where account_id = a.id order by created_at desc limit 1
+      ) s on true
+      left join (select account_id, count(*) cnt from instances group by account_id) i
+        on i.account_id = a.id
+      order by a.created_at
+  `);
+  const header = ["Email","Firma","Ansprechperson","Strasse","PLZ","Ort","Land",
+    "Kunde_seit","Plan","Abo_Status","Periode_bis","Preis_CHF_mtl","Instanzen"];
+  const lines = [header.join(";")];
+  for (const r of rows) {
+    const price = getPlan(r.plan)?.priceChf ?? "";
+    lines.push([
+      r.email, r.company, r.contact_name, r.street, r.zip, r.city, r.country,
+      r.created_at ? new Date(r.created_at).toISOString().slice(0, 10) : "",
+      r.plan || "", r.sub_status || "", r.current_period_end ? new Date(r.current_period_end).toISOString().slice(0, 10) : "",
+      r.sub_status === "active" ? price : "", r.instance_count,
+    ].map(csvCell).join(";"));
+  }
+  const csv = "" + lines.join("\r\n");
+  res.setHeader("Content-Type", "text/csv; charset=utf-8");
+  res.setHeader("Content-Disposition", `attachment; filename="rapport-kunden-${new Date().toISOString().slice(0,10)}.csv"`);
+  res.send(csv);
+});
+
 // — Instanzen eines Kontos (Kurzform) —
 adminRouter.get("/accounts/:id/instances", async (req, res) => {
   const { rows } = await query(