Countly Migration 1.8.1

Improve progress calculations and merge cluster data for migration stats

- Updated `/stats` route to compute collection progress dynamically with cluster-wide data integration.
- Enhanced completed collection recovery using Redis aggregates to include expired progress data.
- Refactored progress tracking to handle local and merged cluster metrics for accurate reporting.
- Improved `CollectionOrchestrator` by adding fallback logic for `batchSeq` initialization.