perf: optimize task history retention purge with parallel processing

- Add parallel metadata reads with 50 concurrent operations
- Add parallel deletions with 10 concurrent operations
- Batch read all metadata first, then filter, then delete
- Reduce retry attempts from 3 to 1 (remove aggressive retries with sleeps)
- Extract helper functions for cleaner code (readTaskMetadata, pathExists, removeDir)

This significantly improves purge performance for users with many tasks (~9000+).

Author: hannesrudolph