feat(backend): handle processing errors and add PWA routes

- Block file downloads and previews with a 424 StatusFailedDependency if file processing failed or the box has issues.
- Register routes for `/service-worker.js` and `/share-target` to support PWA features.
- Update README.md with an AI usage disclosure.

backend/libs/jobs/thumbnails.go

@@ -45,6 +45,10 @@ func GenerateThumbnailsForBoxAsync(uploadService *services.UploadService, logger
 			logger.Warn("thumbnail box lookup failed", "source", "thumbnail", "severity", "warn", "code", 4204, "box_id", boxID, "error", err.Error())
 			return
 		}
+		if services.BoxHasTrouble(box) {
+			logger.Warn("thumbnail one-shot skipped trouble box", "source", "thumbnail", "severity", "warn", "code", 4206, "box_id", boxID, "error", services.BoxTroubleReason(box))
+			return
+		}
 
 		result, err := generateMissingThumbnailsForBox(uploadService, logger, box)
 		if err != nil {
@@ -91,6 +95,9 @@ func generateMissingThumbnails(uploadService *services.UploadService, logger *sl
 		if !box.ExpiresAt.After(now) {
 			continue
 		}
+		if services.BoxHasTrouble(box) {
+			continue
+		}
 
 		boxResult, err := generateMissingThumbnailsForBox(uploadService, logger, box)
 		result.Scanned += boxResult.Scanned
@@ -109,10 +116,16 @@ func generateMissingThumbnailsForBox(uploadService *services.UploadService, logg
 	if !box.ExpiresAt.After(time.Now().UTC()) {
 		return result, nil
 	}
+	if services.BoxHasTrouble(box) {
+		return result, nil
+	}
 
 	changed := false
 	for i := range box.Files {
 		file := &box.Files[i]
+		if file.Processing || services.FileHasTrouble(*file) {
+			continue
+		}
 		needsPrimary := file.Thumbnail == "" && needsThumbnail(*file)
 		needsScenes := file.SceneThumbnail == "" && needsVideoScenes(*file)
 		needsArchive := !archiveListingCurrent(*file) && needsArchiveListing(*file)
@@ -206,6 +219,15 @@ func GenerateArchiveListingForFile(uploadService *services.UploadService, box se
 }
 
 func generateThumbnail(uploadService *services.UploadService, box services.Box, file services.File) (string, error) {
+	if services.BoxHasTrouble(box) {
+		return "", fmt.Errorf("box is marked as trouble: %s", services.BoxTroubleReason(box))
+	}
+	if file.Processing {
+		return "", fmt.Errorf("file is still processing")
+	}
+	if services.FileHasTrouble(file) {
+		return "", fmt.Errorf("file processing failed: %s", file.ProcessingError)
+	}
 	thumbnailName := "@thumb@" + file.ID + ".jpg"
 	object, err := uploadService.OpenFileObject(context.Background(), box, file)
 	if err != nil {
@@ -244,6 +266,15 @@ func generateVideoScenesThumbnail(uploadService *services.UploadService, box ser
 	if !needsVideoScenes(file) {
 		return "", nil
 	}
+	if services.BoxHasTrouble(box) {
+		return "", fmt.Errorf("box is marked as trouble: %s", services.BoxTroubleReason(box))
+	}
+	if file.Processing {
+		return "", fmt.Errorf("file is still processing")
+	}
+	if services.FileHasTrouble(file) {
+		return "", fmt.Errorf("file processing failed: %s", file.ProcessingError)
+	}
 	sceneName := "@scene@" + file.ID + ".jpg"
 	object, err := uploadService.OpenFileObject(context.Background(), box, file)
 	if err != nil {
@@ -263,6 +294,15 @@ func generateArchiveListing(uploadService *services.UploadService, box services.
 	if !needsArchiveListing(file) {
 		return "", nil
 	}
+	if services.BoxHasTrouble(box) {
+		return "", fmt.Errorf("box is marked as trouble: %s", services.BoxTroubleReason(box))
+	}
+	if file.Processing {
+		return "", fmt.Errorf("file is still processing")
+	}
+	if services.FileHasTrouble(file) {
+		return "", fmt.Errorf("file processing failed: %s", file.ProcessingError)
+	}
 	listingName := "@archive@" + file.ID + ".json"
 	object, err := uploadService.OpenFileObject(context.Background(), box, file)
 	if err != nil {

backend/libs/jobs/thumbnails_test.go

@@ -50,6 +50,36 @@ func TestGenerateMissingThumbnailsForBox(t *testing.T) {
 	}
 }
 
+func TestGenerateMissingThumbnailsForTroubleBoxSkipsWork(t *testing.T) {
+	service := newThumbnailTestUploadService(t)
+	result := createThumbnailTestBox(t, service)
+	box, err := service.GetBox(result.BoxID)
+	if err != nil {
+		t.Fatalf("GetBox returned error: %v", err)
+	}
+	box.Trouble = true
+	box.TroubleReason = "storage backend failed"
+	if err := service.SaveBox(box); err != nil {
+		t.Fatalf("SaveBox returned error: %v", err)
+	}
+
+	jobResult, err := generateMissingThumbnailsForBox(service, slog.New(slog.NewTextHandler(io.Discard, nil)), box)
+	if err != nil {
+		t.Fatalf("generateMissingThumbnailsForBox returned error: %v", err)
+	}
+	if jobResult != (ThumbnailJobResult{}) {
+		t.Fatalf("job result = %+v, want no work for trouble box", jobResult)
+	}
+
+	updated, err := service.GetBox(result.BoxID)
+	if err != nil {
+		t.Fatalf("GetBox after job returned error: %v", err)
+	}
+	if updated.Files[0].Thumbnail != "" {
+		t.Fatalf("thumbnail was generated for trouble box: %+v", updated.Files[0])
+	}
+}
+
 func TestCreateTextThumbnailRendersMarkdownAsJPEG(t *testing.T) {
 	data, err := createTextThumbnail(services.File{
 		Name:        "notes.md",