Notify when attachment download succeeds, fix deadlock in attachment store

kordophoned/src/main.rs

@@ -98,6 +98,16 @@ async fn main() {
                             0
                         });
                 }
+
+                Signal::AttachmentDownloaded(attachment_id) => {
+                    log::debug!("Sending signal: AttachmentDownloaded for attachment {}", attachment_id);
+                    dbus_registry
+                        .send_signal(interface::OBJECT_PATH, DbusSignals::AttachmentDownloadCompleted { attachment_id })
+                        .unwrap_or_else(|_| {
+                            log::error!("Failed to send signal");
+                            0
+                        });
+                }
             }
         }
     });