feat(mail): Microsoft 365 sending via OAuth 2.0 (client credentials)

Sending no longer needs a user password for Microsoft 365. When
SMTP_OAUTH_TENANT_ID, SMTP_OAUTH_CLIENT_ID and SMTP_OAUTH_CLIENT_SECRET
are set, the shop fetches an app-only token from Entra ID and signs in to
smtp.office365.com with XOAUTH2. This works with security defaults and
MFA, survives the end of SMTP basic auth (default off from end of
December 2026) and limits the app to the one mailbox Exchange grants it
(SMTP.SendAsApp + Add-MailboxPermission). Password login stays available
for other providers; a partial OAuth setup never silently falls back.

- lib/ms365-oauth.ts: token cached until 5 minutes before expiry,
  concurrent requests share one fetch, forced refresh after a server
  rejection, 15 s timeout, tenant id validated before it enters the URL,
  client secret redacted from every error.
- lib/smtp-diagnose.ts: explains AADSTS7000215 (secret ID instead of
  value), 7000222 (expired), 700016 (unknown app), 90002/900023 (tenant)
  and maps a rejected XOAUTH2 login to missing mailbox access.
- Admin e-mail test shows the auth mode and lists missing settings.
- k8s manifests, preflight check and README (step-by-step Entra and
  Exchange setup) updated; misplaced SMTP_PORT comment moved.

Verified against Entra ID and smtp.office365.com with fake values:
AADSTS90002 is classified as tenant error without leaking the secret,
an invalid token yields 535 5.7.3 and is classified as mailbox access.

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

k8s/02-secret.example.yaml

@@ -72,9 +72,23 @@ stringData:
   # keine Bestellbestaetigung, keine Versandmeldung, keine
   # Passwort-Zuruecksetzung. Funktioniert mit jedem SMTP-Anbieter; Port und
   # Absenderadresse stehen als SMTP_PORT/SMTP_FROM in 01-configmap.yaml.
+  #
+  # SMTP_USER ist immer das Postfach, mit dem gesendet wird (z. B.
+  # [email protected]) — und sollte dieselbe Adresse wie SMTP_FROM sein.
+  #
+  # Microsoft 365 mit OAuth 2.0 (empfohlen): SMTP_OAUTH_CLIENT_SECRET setzen,
+  # dazu SMTP_OAUTH_TENANT_ID/SMTP_OAUTH_CLIENT_ID in 01-configmap.yaml.
+  # SMTP_HOST darf leer bleiben (Standard smtp.office365.com), SMTP_PASSWORD
+  # wird nicht gebraucht. Das Secret laeuft ab (hoechstens 24 Monate) —
+  # Ablaufdatum im Kalender eintragen.
+  #
+  # Andere Anbieter: SMTP_HOST und SMTP_PASSWORD setzen,
+  # SMTP_OAUTH_CLIENT_SECRET leer lassen.
   SMTP_HOST: "CHANGE-ME"
   SMTP_USER: "CHANGE-ME"
   SMTP_PASSWORD: "CHANGE-ME"
+  # "Wert" (nicht "Geheimnis-ID") des geheimen Clientschluessels der App
+  SMTP_OAUTH_CLIENT_SECRET: ""
 
   # ---------------------------------------------------------------------
   # EMPFOHLEN — der Shop laeuft auch ohne, aber ungeschuetzt