fix(stats): preserve pruned first_seen and stamp seen_ms at observation

- Record occurrence seen_ms from the observation time instead of flush time
- On session delete, only move first_seen when removed timestamps prove it was deleted; keep earlier dates from retention-pruned or legacy history
- Exact refresh fallback keeps subtraction-exact frequency and preserved first_seen
- Add tests for pruned history, legacy rows, and delayed flushes; update docs and change fragments

changes/stats-overview-refresh-docs.md

@@ -2,3 +2,4 @@ type: docs
 area: stats
 
 - Documented the automatic Overview refresh after deleting sessions or session groups and retrying failed refreshes.
+- Documented discovery-date preservation after retention pruning and the handling of older write-time timestamps.