Notify when attachment download succeeds, fix deadlock in attachment store

kordophoned/src/daemon/signals.rs

@@ -7,4 +7,9 @@ pub enum Signal {
     /// Parameters:
     /// - conversation_id: The ID of the conversation that was updated.
     MessagesUpdated(String),
+
+    /// Emitted when an attachment has been downloaded.
+    /// Parameters:
+    /// - attachment_id: The ID of the attachment that was downloaded.
+    AttachmentDownloaded(String),
 }