Feat: Add configurable leverage settings for BTC/ETH and altcoins (v2.0.3)

- Add LeverageConfig struct with btc_eth_leverage and altcoin_leverage fields
- Set default leverage to 5x (safe for Binance subaccounts)
- Add validation warnings for leverage >5x (subaccount restrictions)
- Update config.json.example with leverage configuration

Breaking changes:
- None (backward compatible with default 5x leverage)

Migration:
- Existing configs will auto-default to 5x leverage (safe)
- Main accounts can increase to 20x (altcoins) or 50x (BTC/ETH) in config.json
- Subaccounts must keep leverage ≤5x to avoid trade failures

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

Co-Authored-By: tinkle-community <tinklefund@gmail.com>

config.json.example

@@ -13,6 +13,10 @@
       "scan_interval_minutes": 3
     }
   ],
+  "leverage": {
+    "btc_eth_leverage": 5,
+    "altcoin_leverage": 5
+  },
   "use_default_coins": true,
   "coin_pool_api_url": "",
   "oi_top_api_url": "",