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/indodax/trader.go

@@ -176,6 +176,13 @@ func (t *IndodaxTrader) doPrivateRequest(params url.Values) ([]byte, error) {
 	if resp.StatusCode == http.StatusTooManyRequests {
 		return nil, fmt.Errorf("rate limit exceeded, please try again later")
 	}
+	if resp.StatusCode < 200 || resp.StatusCode >= 300 {
+		snippet := string(data)
+		if len(snippet) > 256 {
+			snippet = snippet[:256] + "..."
+		}
+		return nil, fmt.Errorf("Indodax API HTTP error (status %d): %s", resp.StatusCode, snippet)
+	}
 
 	// Parse response to check success
 	var apiResp IndodaxResponse