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/aster/trader_test.go

@@ -1,7 +1,6 @@
 package aster
 
 import (
-	"context"
 	"encoding/json"
 	"io"
 	"net/http"
@@ -194,7 +193,6 @@ func NewAsterTraderTestSuite(t *testing.T) *AsterTraderTestSuite {
 
 	// Create mock trader using mock server's URL
 	traderInstance := &AsterTrader{
-		ctx:             context.Background(),
 		user:            "0x1234567890123456789012345678901234567890",
 		signer:          "0xabcdefabcdefabcdefabcdefabcdefabcdefabcd",
 		privateKey:      privateKey,