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

api/agent_routes.go

@@ -11,6 +11,7 @@ import (
 func (s *Server) RegisterAgentHandler(h *agent.WebHandler) {
 	// Chat requires auth — can trigger trades and access account data
 	s.router.POST("/api/agent/chat", s.authMiddleware(), gin.WrapF(h.HandleChat))
+	s.router.POST("/api/agent/chat/stream", s.authMiddleware(), gin.WrapF(h.HandleChatStream))
 	// Public endpoints — read-only market data
 	s.router.GET("/api/agent/health", gin.WrapF(h.HandleHealth))
 	s.router.GET("/api/agent/klines", gin.WrapF(h.HandleKlines))