Get queue statistics

**Queue Statistics** Retrieves comprehensive statistics about the notification queue system. **Metrics:** - Queue status counts (pending, processing, completed, etc.) - Processor status (running, active jobs, concurrency) - Health indicators (queue health, processor health) **Use Cases:** - Monitoring queue performance - Health checks - Capacity planning - Troubleshooting **Example Response:** ``` { "success": true, "data": { "queue": { "total": 150, "pending": 25, "processing": 3, "completed": 120, "failed": 2, "retrying": 0, "expired": 0, "cancelled": 0 }, "processor": { "isRunning": true, "isProcessing": true, "activeJobs": 3, "maxConcurrency": 10 }, "health": { "queueHealthy": true, "processorHealthy": true, "overallHealthy": true } } } ```