feat: real SSE streaming for agent chat + data cleanup on startup

SSE Streaming:
- Add POST /api/agent/chat/stream endpoint with real SSE streaming
- Agent.HandleMessageStream() streams final LLM response via onEvent callback
- Tool-calling rounds use non-streaming; tool status sent as 'tool' events
- Frontend uses ReadableStream API instead of fake word-by-word setTimeout
- SSE events: tool (tool execution), delta (text chunk), done (complete), error

Data Maintenance:
- Add cleanupOldData() on server startup (decisions >90d, equity >180d)
- Add CleanOldRecords to DecisionStore and EquityStore
- Add store-level logger for cleanup operations

Exchange Traders:
- Add GetAccountInfo() to Bitget, Bybit, Indodax, KuCoin, Lighter, OKX traders
- Remove unused imports in Aster/Lighter traders

trader/bitget/trader.go

@@ -193,6 +193,15 @@ func (t *BitgetTrader) doRequest(method, path string, body interface{}) ([]byte,
 	}
 	defer resp.Body.Close()
 
+	if resp.StatusCode < 200 || resp.StatusCode >= 300 {
+		errBody, _ := safe.ReadAllLimited(resp.Body)
+		snippet := string(errBody)
+		if len(snippet) > 256 {
+			snippet = snippet[:256] + "..."
+		}
+		return nil, fmt.Errorf("Bitget API HTTP error (status %d): %s", resp.StatusCode, snippet)
+	}
+
 	respBody, err := safe.ReadAllLimited(resp.Body)
 	if err != nil {
 		return nil, fmt.Errorf("failed to read response: %w", err)