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.service.ts

@@ -42,6 +42,10 @@ export class MessagesService {
 		return this.messagesRepository.findAllConversations(userId);
 	}
 
+	async getUnreadCount(userId: string) {
+		return this.messagesRepository.countUnreadMessages(userId);
+	}
+
 	async getConversationWithUser(userId: string, targetUserId: string) {
 		return this.messagesRepository.findConversationBetweenUsers(
 			userId,