feat(liquidity): Event-based pipeline with WebSocket order monitoring

[TaprootFreak]

Summary

• Adds real-time order completion detection via CCXT Pro WebSockets for Kraken, Binance, MEXC, and XT exchanges
• Implements event-driven pipeline continuation to eliminate polling delays
• Adds atomic database operations to prevent race conditions between event handlers and cron jobs

Changes

New Files

• ExchangeWebSocketService: Manages WebSocket connections to exchanges for real-time order updates
• OrderCompletionService: Centralized order completion handling with WebSocket + polling fallback

Modified Files

• LiquidityManagementPipelineService: Subscribes to order completion events for immediate pipeline continuation
• LiquidityManagementOrderRepository: Atomic tryClaimForExecution and saveExecutionResult methods
• LiquidityManagementPipelineRepository: Atomic tryContinuePipeline with pessimistic locking

Performance Improvement

Metric	Before	After
Delay after order completion	up to 60s	~0ms (WebSocket) / ~5s (polling)
Cron frequency (fallback)	1 min	5 min

Architecture

Order completes on Exchange
         │
         ▼
WebSocket Event (CCXT Pro)
         │
         ▼
OrderCompletionService detects completion
         │
         ▼
Event emitted via RxJS Subject
         │
         ▼
PipelineService continues immediately
         │
    (Fallback: 5s polling + 5min cron)

Test plan

• Verify WebSocket connections initialize correctly for configured exchanges
• Test order completion detection via WebSocket for BUY orders
• Test order completion detection via WebSocket for SELL orders
• Verify polling fallback works when WebSocket is unavailable
• Test race condition protection (concurrent event + cron)
• Verify graceful shutdown closes all WebSocket connections