Added two monitoring improvements to the workspace migration runner:
1. **Unique run IDs for timer labels:** All `this.logger.time`/`timeEnd` calls now include a unique `runId` (workspace prefix + random suffix) to prevent `console.time` label collisions that were producing misleading timing data and noisy Node.js warnings in production logs.
2. **Concurrent execution detection:** Added an `activeRunsByWorkspace` Map that tracks how many migration runner executions are active per workspace. When a new run starts while another is already in-flight for the same workspace, a warning is logged with the workspace ID and active run count. This makes concurrent execution issues immediately visible without requiring database-level monitoring.