feat: update local authentication mode to require a non-placeholder token of at least 50 characters

.env.example

@@ -17,7 +17,8 @@ DB_AUTO_MIGRATE=true
 LOG_LEVEL=INFO
 REQUEST_LOG_SLOW_MS=1000
 AUTH_MODE=local
-LOCAL_AUTH_TOKEN=change-me
+# REQUIRED when AUTH_MODE=local (must be non-placeholder and at least 50 chars).
+LOCAL_AUTH_TOKEN=
 
 # --- frontend settings ---
 # REQUIRED: Public URL used by the browser to reach the API.