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/libs/handlers/download.go

@@ -47,7 +47,9 @@ type fileView struct {
 	URL          string
 	DownloadURL  string
 	ThumbnailURL string
+	SceneURL     string
 	HasThumbnail bool
+	HasScene     bool
 	IconURL      string
 	IconRetroURL string
 	ReactURL     string
@@ -345,6 +347,43 @@ func (a *App) Thumbnail(w http.ResponseWriter, r *http.Request) {
 	http.ServeContent(w, r, file.ID+"-thumbnail.jpg", object.ModTime, readSeekCloser(object.Body))
 }
 
+func (a *App) VideoScenesPreview(w http.ResponseWriter, r *http.Request) {
+	w.Header().Set("X-Robots-Tag", "noindex, nofollow, noarchive")
+	box, file, ok := a.loadFileForRequest(w, r)
+	if !ok {
+		return
+	}
+	if !jobs.NeedsVideoScenes(file) {
+		http.NotFound(w, r)
+		return
+	}
+	if a.uploadService.IsProtected(box) && box.Obfuscate && !a.isBoxUnlocked(r, box) {
+		a.servePlaceholderThumbnail(w, r)
+		return
+	}
+
+	object, err := a.uploadService.OpenSceneThumbnailObject(r.Context(), box, file)
+	if err != nil {
+		if scene := a.generateMissingVideoScenesForRequest(r, box, file); scene != "" {
+			file.SceneThumbnail = scene
+			object, err = a.uploadService.OpenSceneThumbnailObject(r.Context(), box, file)
+			if err == nil {
+				defer object.Body.Close()
+				w.Header().Set("Content-Type", "image/jpeg")
+				w.Header().Set("Cache-Control", "public, max-age=604800, immutable")
+				http.ServeContent(w, r, file.ID+"-scenes.jpg", object.ModTime, readSeekCloser(object.Body))
+				return
+			}
+		}
+		a.servePlaceholderThumbnail(w, r)
+		return
+	}
+	defer object.Body.Close()
+	w.Header().Set("Content-Type", "image/jpeg")
+	w.Header().Set("Cache-Control", "public, max-age=604800, immutable")
+	http.ServeContent(w, r, file.ID+"-scenes.jpg", object.ModTime, readSeekCloser(object.Body))
+}
+
 func (a *App) generateMissingThumbnailForRequest(r *http.Request, box services.Box, file services.File) string {
 	if file.Thumbnail != "" || !jobs.NeedsThumbnail(file) {
 		return ""
@@ -369,6 +408,30 @@ func (a *App) generateMissingThumbnailForRequest(r *http.Request, box services.B
 	return thumbnail
 }
 
+func (a *App) generateMissingVideoScenesForRequest(r *http.Request, box services.Box, file services.File) string {
+	if file.SceneThumbnail != "" || !jobs.NeedsVideoScenes(file) {
+		return ""
+	}
+	scene, err := jobs.GenerateVideoScenesForFile(a.uploadService, box, file)
+	if err != nil || scene == "" {
+		if err != nil {
+			a.logger.Warn("on-demand video scenes preview generation failed", withRequestLogAttrs(r, "source", "thumbnail", "severity", "warn", "code", 4105, "box_id", box.ID, "file_id", file.ID, "error", err.Error())...)
+		}
+		return ""
+	}
+	for i := range box.Files {
+		if box.Files[i].ID == file.ID {
+			box.Files[i].SceneThumbnail = scene
+			break
+		}
+	}
+	if err := a.uploadService.SaveBox(box); err != nil {
+		a.logger.Warn("on-demand video scenes preview metadata save failed", withRequestLogAttrs(r, "source", "thumbnail", "severity", "warn", "code", 4106, "box_id", box.ID, "file_id", file.ID, "error", err.Error())...)
+		return ""
+	}
+	return scene
+}
+
 // servePlaceholderThumbnail serves the fallback image with no-store so the
 // browser re-requests on the next load and picks up the real thumbnail as soon
 // as it has been generated.
@@ -513,7 +576,9 @@ func (a *App) fileViewWithReactions(box services.Box, file services.File, reacti
 		URL:          fmt.Sprintf("/d/%s/f/%s", box.ID, file.ID),
 		DownloadURL:  fmt.Sprintf("/d/%s/f/%s/download", box.ID, file.ID),
 		ThumbnailURL: fmt.Sprintf("/d/%s/thumb/%s", box.ID, file.ID),
+		SceneURL:     fmt.Sprintf("/d/%s/scene/%s", box.ID, file.ID),
 		HasThumbnail: file.Thumbnail != "" || jobs.NeedsThumbnail(file),
+		HasScene:     file.SceneThumbnail != "" || jobs.NeedsVideoScenes(file),
 		IconURL:      fileIconURL("standard", icon.Standard),
 		IconRetroURL: fileIconURL("retro", icon.Retro),
 		ReactURL:     fmt.Sprintf("/d/%s/f/%s/react", box.ID, file.ID),