fix(backend): invalidate thumbnail cache on replace and permanent delete

Replacing a file's content left the old {id}_thumb.jpg / {id}_preview.jpg
in the cache, and the cache-hit fast path kept serving the stale image
forever; permanent deletion left those files orphaned. FileStorage gains
InvalidateCache, which Replace and PermanentDelete now call.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/internal/port/storage.go

@@ -21,6 +21,10 @@ type FileStorage interface {
 	// Delete removes the file content from storage.
 	Delete(ctx context.Context, id uuid.UUID) error
 
+	// InvalidateCache removes any cached thumbnail/preview for the file so they
+	// are regenerated from the current content on next request.
+	InvalidateCache(ctx context.Context, id uuid.UUID) error
+
 	// Thumbnail opens the pre-generated thumbnail (JPEG). Returns ErrNotFound
 	// if the thumbnail has not been generated yet.
 	Thumbnail(ctx context.Context, id uuid.UUID) (io.ReadCloser, error)

backend/internal/service/file_service.go

@@ -345,6 +345,7 @@ func (s *FileService) PermanentDelete(ctx context.Context, id uuid.UUID) error {
 		return err
 	}
 	_ = s.storage.Delete(ctx, id)
+	_ = s.storage.InvalidateCache(ctx, id)
 
 	objType := fileObjectType
 	_ = s.audit.Log(ctx, "file_permanent_delete", &objType, &id, nil)
@@ -383,6 +384,8 @@ func (s *FileService) Replace(ctx context.Context, id uuid.UUID, p UploadParams)
 	if _, err := s.storage.Save(ctx, id, bytes.NewReader(data)); err != nil {
 		return nil, fmt.Errorf("FileService.Replace: save to storage: %w", err)
 	}
+	// Drop stale thumbnail/preview so they regenerate from the new content.
+	_ = s.storage.InvalidateCache(ctx, id)
 
 	patch := &domain.File{
 		MIMEType:      mime.Name,

backend/internal/storage/disk.go

@@ -109,6 +109,17 @@ func (s *DiskStorage) Delete(_ context.Context, id uuid.UUID) error {
 	return nil
 }
 
+// InvalidateCache removes the cached thumbnail and preview for id, if present,
+// so they are regenerated from the current file content on the next request.
+func (s *DiskStorage) InvalidateCache(_ context.Context, id uuid.UUID) error {
+	for _, p := range []string{s.thumbCachePath(id), s.previewCachePath(id)} {
+		if err := os.Remove(p); err != nil && !os.IsNotExist(err) {
+			return fmt.Errorf("storage.InvalidateCache remove %q: %w", p, err)
+		}
+	}
+	return nil
+}
+
 // Thumbnail returns a JPEG that fits within the configured max width×height
 // (never upscaled, never cropped). Generated on first call and cached.
 // Video files are thumbnailed via ffmpeg; other non-image files get a placeholder.