feat(admin): Zahlungsausfaelle + Instanz-Health-Check

- /admin/stats: pastDueSubscriptions (Abos mit fehlgeschlagener Zahlung)
- /admin/health: pingt aktive Instanz-URLs (HEAD, 4s Timeout) -> up/down;
  im MOCK-Modus ehrlich 'unknown' statt fake 'up'

E2E: past_due fliesst in stats, health gibt im Mock 'unknown'.

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

server/routes/admin.js

@@ -5,7 +5,7 @@ import { Router } from "express";
 import { one, query } from "../db.js";
 import { requireAdmin, signAdminToken } from "../auth.js";
 import { getPlan } from "../plans.js";
-import { env } from "../env.js";
+import { env, provisioningMock } from "../env.js";
 
 export const adminRouter = Router();
 
@@ -29,6 +29,8 @@ adminRouter.get("/stats", async (_req, res) => {
   const instances = await one("select count(*)::int n from instances");
   const activeInst = await one("select count(*)::int n from instances where status = 'active'");
   const suspendedInst = await one("select count(*)::int n from instances where status = 'suspended'");
+  // Zahlungsprobleme: Abos in past_due (fehlgeschlagene Zahlung) — braucht Aufmerksamkeit.
+  const pastDue = await one("select count(*)::int n from subscriptions where status = 'past_due'");
 
   // MRR + Verteilung nach Plan (nur aktive Abos).
   const { rows: subs } = await query("select plan from subscriptions where status = 'active'");
@@ -147,6 +149,47 @@ adminRouter.get("/accounts/:id/instances", async (req, res) => {
   res.json({ instances: rows });
 });
 
+// — Health-Check aller aktiven Instanzen —
+// Pingt jede Instanz-URL (HEAD, kurzes Timeout) und meldet up/down. Im
+// MOCK-Modus (provisioningMock) sind die URLs synthetisch → Status "unknown"
+// statt fake "up", damit das Cockpit ehrlich bleibt.
+adminRouter.get("/health", async (_req, res) => {
+  const { rows } = await query(
+    "select id, studio_slug, label, instance_url, status from instances where status = 'active'"
+  );
+
+  if (provisioningMock) {
+    return res.json({
+      mock: true,
+      checked: 0,
+      instances: rows.map((i) => ({ ...i, health: "unknown" })),
+    });
+  }
+
+  async function ping(url) {
+    const ctrl = new AbortController();
+    const t = setTimeout(() => ctrl.abort(), 4000);
+    try {
+      const r = await fetch(url, { method: "HEAD", signal: ctrl.signal, redirect: "manual" });
+      return r.status < 500 ? "up" : "down";
+    } catch {
+      return "down";
+    } finally {
+      clearTimeout(t);
+    }
+  }
+
+  const checked = await Promise.all(
+    rows.map(async (i) => ({ ...i, health: await ping(i.instance_url) }))
+  );
+  res.json({
+    mock: false,
+    checked: checked.length,
+    down: checked.filter((i) => i.health === "down").length,
+    instances: checked,
+  });
+});
+
 // — Instanz sperren / reaktivieren —
 adminRouter.post("/instances/:id/:action", async (req, res) => {
   const { id, action } = req.params;