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/CachedFrame.cs

@@ -0,0 +1,28 @@
+namespace BomLocalService.Models;
+
+/// <summary>
+/// Base model representing a single cached frame image.
+/// Extended by type-specific frame models (RadarFrame, etc.).
+/// </summary>
+public class CachedFrame
+{
+    /// <summary>
+    /// Frame index within the frameset (0-based).
+    /// </summary>
+    public int FrameIndex { get; set; }
+    
+    /// <summary>
+    /// Full file system path to the frame image file (server-side only).
+    /// </summary>
+    public string ImagePath { get; set; } = string.Empty;
+    
+    /// <summary>
+    /// URL endpoint to retrieve this frame image.
+    /// </summary>
+    public string ImageUrl { get; set; } = string.Empty;
+    
+    /// <summary>
+    /// The data type this frame belongs to.
+    /// </summary>
+    public CachedDataType DataType { get; set; }
+}