docs: update .env.example to recommend keygen.sh

Replace manual secret generation instructions with keygen.sh recommendation.

Before:
  # Secrets generieren:
  #   openssl rand -base64 32

After:
  # EMPFOHLEN: Automatische Konfiguration mit:
  #   ./keygen.sh

This makes it clearer that keygen.sh is the preferred way to set up
the environment, as it generates all secrets and creates a complete
.env file automatically.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

.env.example

@@ -1,11 +1,11 @@
 # ===========================================
 # SafeDocs Portal - Environment Configuration
 # ===========================================
-# Kopieren nach .env und Werte anpassen:
+# EMPFOHLEN: Automatische Konfiguration mit:
-#   cp .env.example .env
+#   ./keygen.sh
 #
-# Secrets generieren:
+# Alternativ: Manuell kopieren und anpassen:
-#   openssl rand -base64 32
+#   cp .env.example .env
 # ===========================================
 
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