fix: add beta_mode to config.json.example

Add missing beta_mode configuration option to the example config file.
This ensures Docker builds and new deployments include the beta_mode setting.

🤖 Generated with [Claude Code](https://claude.ai/code)

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

config.json.example

@@ -1,5 +1,6 @@
 {
   "admin_mode": true,
+  "beta_mode": false,
   "leverage": {
     "btc_eth_leverage": 5,
     "altcoin_leverage": 5