feat: add unread message count API

- Added `GET /messages/unread-count` endpoint to retrieve the count of unread messages for a user.
- Implemented `getUnreadCount` method in `MessagesService` and `MessagesRepository`.
- Updated frontend service to support fetching unread message count via API.

backend/src/messages/messages.controller.ts

@@ -22,6 +22,11 @@ export class MessagesController {
 		return this.messagesService.getConversations(req.user.sub);
 	}
 
+	@Get("unread-count")
+	getUnreadCount(@Req() req: AuthenticatedRequest) {
+		return this.messagesService.getUnreadCount(req.user.sub);
+	}
+
 	@Get("conversations/with/:userId")
 	getConversationWithUser(
 		@Req() req: AuthenticatedRequest,