feat(backend): implement on-demand thumbnail generation

When a thumbnail is requested but not yet available, attempt to generate
it synchronously on-demand instead of immediately falling back to the
placeholder image.

- Export thumbnail generation helpers from the jobs package.
- Update the Thumbnail handler to trigger on-demand generation if the
  thumbnail object is missing.
- Save the updated box metadata with the new thumbnail reference.
- Fall back to the placeholder image only if on-demand generation fails.

backend/libs/handlers/download.go

@@ -15,6 +15,7 @@ import (
 	"time"
 
 	"warpbox.dev/backend/libs/helpers"
+	"warpbox.dev/backend/libs/jobs"
 	"warpbox.dev/backend/libs/services"
 	"warpbox.dev/backend/libs/web"
 )
@@ -319,6 +320,17 @@ func (a *App) Thumbnail(w http.ResponseWriter, r *http.Request) {
 
 	object, err := a.uploadService.OpenThumbnailObject(r.Context(), box, file)
 	if err != nil {
+		if thumbnail := a.generateMissingThumbnailForRequest(r, box, file); thumbnail != "" {
+			file.Thumbnail = thumbnail
+			object, err = a.uploadService.OpenThumbnailObject(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+"-thumbnail.jpg", object.ModTime, readSeekCloser(object.Body))
+				return
+			}
+		}
 		// The thumbnail isn't generated yet (background job pending). Serve the
 		// placeholder but mark it non-cacheable, otherwise the browser would
 		// keep showing the placeholder until a hard refresh once the real
@@ -333,6 +345,30 @@ 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) generateMissingThumbnailForRequest(r *http.Request, box services.Box, file services.File) string {
+	if file.Thumbnail != "" || !jobs.NeedsThumbnail(file) {
+		return ""
+	}
+	thumbnail, err := jobs.GenerateThumbnailForFile(a.uploadService, box, file)
+	if err != nil || thumbnail == "" {
+		if err != nil {
+			a.logger.Warn("on-demand thumbnail generation failed", withRequestLogAttrs(r, "source", "thumbnail", "severity", "warn", "code", 4102, "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].Thumbnail = thumbnail
+			break
+		}
+	}
+	if err := a.uploadService.SaveBox(box); err != nil {
+		a.logger.Warn("on-demand thumbnail metadata save failed", withRequestLogAttrs(r, "source", "thumbnail", "severity", "warn", "code", 4103, "box_id", box.ID, "file_id", file.ID, "error", err.Error())...)
+		return ""
+	}
+	return thumbnail
+}
+
 // 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.