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/CachedDataType.cs

@@ -0,0 +1,16 @@
+namespace BomLocalService.Models;
+
+/// <summary>
+/// Types of cached data available from BOM weather pages.
+/// </summary>
+public enum CachedDataType
+{
+    /// <summary>
+    /// Rain radar images showing historical precipitation (40 min ago → 10 min ago).
+    /// Frames represent a time series and can be joined across cache folders for extended historical slideshows.
+    /// </summary>
+    Radar
+    // Future data types (not yet implemented):
+    // Temperature - forecast images showing current conditions and projected temperatures for the week ahead
+    // Wind - forecast images showing current conditions and projected wind patterns for the week ahead
+}