fix: prevent panics from unsafe type assertions in trading code + add request body limit

Security & Reliability:
- Add requestBodyLimitMiddleware (1MB) to prevent OOM from oversized API payloads
- Fix defer resp.Body.Close() inside loop in getPublicIPFromAPI (connection leak)
- Add posFloat64/posString safe helpers for position map access

Panic Prevention (critical for trading):
- Convert 30+ unsafe type assertions (pos["key"].(type)) to safe comma-ok
  pattern across all exchange traders: OKX, Hyperliquid, Aster, Bybit,
  KuCoin, Gate, Bitget, Binance
- Fix auto_trader_risk.go: drawdown monitor could panic and silently stop
  monitoring, leaving positions unprotected
- Fix auto_trader_decision.go & auto_trader_loop.go: core trading loop
  position parsing now crash-proof
- All trader/ code now has zero unsafe type assertions

Frontend:
- Fix config.ts: rejected promise cached forever on network error (never retries)

trader/bitget/trader_orders.go

@@ -129,7 +129,7 @@ func (t *BitgetTrader) CloseLong(symbol string, quantity float64) (map[string]in
 		}
 		for _, pos := range positions {
 			if pos["symbol"] == symbol && pos["side"] == "long" {
-				quantity = pos["positionAmt"].(float64)
+				quantity, _ = pos["positionAmt"].(float64)
 				break
 			}
 		}
@@ -192,7 +192,7 @@ func (t *BitgetTrader) CloseShort(symbol string, quantity float64) (map[string]i
 		}
 		for _, pos := range positions {
 			if pos["symbol"] == symbol && pos["side"] == "short" {
-				quantity = pos["positionAmt"].(float64)
+				quantity, _ = pos["positionAmt"].(float64)
 				break
 			}
 		}