feat(admin): Betreiber-Panel (/api/admin) mit is_admin-Flag

- 0003_admin.sql: accounts.is_admin
- auth.js: ensureAdminFlag (Konto = ADMIN_EMAIL wird auto-promoted),
  is_admin im JWT, requireAdmin-Middleware (prüft DB autoritativ)
- routes/admin.js: GET /stats (Kunden/Abos/Instanzen/MRR), GET /accounts,
  GET /accounts/:id/instances, POST /instances/:id/{suspend,reactivate}
- register/login + /account/me liefern is_admin
- ADMIN_EMAIL in .env.example

E2E: Admin-Promotion, Kunde→403, Stats (2 Kunden/MRR 49), Kundenliste.

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

server/routes/account.js

@@ -12,7 +12,7 @@ const PROFILE_FIELDS = ["company", "contact_name", "street", "zip", "city", "cou
 // ── Konto-Übersicht: Konto + Profil + aktuelles Abo + alle Instanzen ─────────
 accountRouter.get("/me", requireAuth, async (req, res) => {
   const account = await one(
-    `select id, email, company, contact_name, street, zip, city, country, phone, created_at
+    `select id, email, company, contact_name, street, zip, city, country, phone, is_admin, created_at
        from accounts where id = $1`,
     [req.account.id]
   );