docs: step-by-step setup guide for secrets, SSO and Microsoft 365 mail

docs/EINRICHTUNG.md walks from nothing to a running shop: tools and
access, generating database/session/admin values, the Entra ID app
registration used for both sign-in and mail (redirect URI, client
secret, SMTP.SendAsApp with admin consent, enterprise app object id),
unlocking the shared mailbox in Exchange Online, Stripe keys and webhook
events, optional services, a complete secret/config map table, rollout,
verification, maintenance (secret expiry) and troubleshooting keyed to
the messages of the admin email test. Linked from README and k8s/README.

Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>

README.md

@@ -2,6 +2,8 @@
 
 Regionale Imkerei-Website und Shop für Wabenhain (Altenmünster) — MVP-Stufe.
 
+> **Neu einrichten?** Alle Secrets, die Microsoft-Anmeldung, den Mailversand über Microsoft 365, Stripe und das Ausrollen beschreibt Schritt für Schritt **[docs/EINRICHTUNG.md](docs/EINRICHTUNG.md)**.
+
 ## Tech-Stack
 
 | Bereich | Technologie |