feat(backend): add video scene preview generation and endpoint

- Register a new route `GET /d/{boxID}/scene/{fileID}` to serve video scene previews.
- Implement the `VideoScenesPreview` handler to serve existing previews or generate them on-demand.
- Add helper functions to analyze video frames (e.g., luma calculation to filter out dark frames) and render the final scene thumbnail.
- Update the `fileView` struct to include scene URL and status fields.

backend/templates/pages/preview.html

@@ -23,7 +23,7 @@
           </a>
         </header>
 
-        <div class="preview-window" data-preview-kind="{{.Data.File.PreviewKind}}" data-file-name="{{.Data.File.Name}}" data-content-type="{{.Data.File.ContentType}}" data-size-bytes="{{.Data.File.SizeBytes}}" data-source-url="{{.Data.DownloadURL}}?inline=1" data-download-url="{{.Data.DownloadURL}}" data-icon-url="{{.Data.File.IconURL}}" data-file-size="{{.Data.File.Size}}">
+        <div class="preview-window" data-preview-kind="{{.Data.File.PreviewKind}}" data-file-name="{{.Data.File.Name}}" data-content-type="{{.Data.File.ContentType}}" data-size-bytes="{{.Data.File.SizeBytes}}" data-source-url="{{.Data.DownloadURL}}?inline=1" data-download-url="{{.Data.DownloadURL}}" data-icon-url="{{.Data.File.IconURL}}" data-file-size="{{.Data.File.Size}}" data-scene-url="{{.Data.File.SceneURL}}">
           <div class="preview-window-titlebar">
             <div>
               <strong data-preview-mode-label>Preview</strong>
@@ -49,6 +49,7 @@
             </div>
             <img class="native-preview native-image-preview" data-image-preview src="{{.Data.DownloadURL}}?inline=1" alt="{{.Data.File.Name}}" hidden>
             <video class="native-preview native-video-preview" data-video-preview src="{{.Data.DownloadURL}}?inline=1" poster="{{.Data.File.ThumbnailURL}}" controls preload="metadata" hidden></video>
+            {{if .Data.File.HasScene}}<img class="native-preview video-scenes-preview" data-video-scenes-preview data-scene-src="{{.Data.File.SceneURL}}" alt="Scenes preview for {{.Data.File.Name}}" hidden>{{end}}
             <audio class="native-preview native-audio-preview" data-browser-audio-preview src="{{.Data.DownloadURL}}?inline=1" controls preload="metadata" hidden></audio>
             <div class="code-preview raw-code-preview" data-raw-preview hidden>
               <pre><code data-raw-output></code></pre>