Fix frame deduplication and chronological ordering in time series endpoint

- Fix scraping to wait for display label to change after step forward, preventing duplicate minutesAgo values within same folder
- Add hashset-based deduplication by AbsoluteObservationTime across cache folders
- Filter frames that are too close together (< 4 minutes apart)
- Preserve original folder structure while pruning filtered frames
- Sort folders by earliest frame time to maintain chronological order
- Add validation to detect folder overlaps that could cause out-of-order frames

Models/CacheUpdateStatus.cs

@@ -50,6 +50,7 @@ public class CacheUpdateStatus
     /// - "Cache is valid, no update needed" - Cache exists and is still fresh
     /// - "Cache is stale, update triggered" - Cache exists but expired, update initiated
     /// - "No cache exists, update triggered" - No cache found, update initiated
+    /// - "Cache update already in progress" - An update is currently running
     /// </summary>
     public string? Message { get; set; }
 }