docs: add CSRF_SECRET to .env.example

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.env.example

@@ -93,6 +93,14 @@ AUTHENTIK_COOKIE_DOMAIN=.localhost
 AUTHENTIK_PORT_HTTP=9000
 AUTHENTIK_PORT_HTTPS=9443
 
+# ======================
+# CSRF Protection
+# ======================
+# CRITICAL: Generate a random secret for CSRF token signing
+# Required in production; auto-generated in development (not persistent across restarts)
+# Command to generate: node -e "console.log(require('crypto').randomBytes(32).toString('hex'))"
+CSRF_SECRET=REPLACE_WITH_64_CHAR_HEX_STRING
+
 # ======================
 # JWT Configuration
 # ======================