feat: add API for fetching direct conversation with a user

- Added `GET /messages/conversations/with/:userId` endpoint in the backend to retrieve direct conversation data.
- Implemented corresponding method in `MessagesService` and `MessagesRepository`.
- Updated the frontend service to support fetching direct conversations via API.

backend/src/messages/messages.controller.ts

@@ -22,6 +22,17 @@ export class MessagesController {
 		return this.messagesService.getConversations(req.user.sub);
 	}
 
+	@Get("conversations/with/:userId")
+	getConversationWithUser(
+		@Req() req: AuthenticatedRequest,
+		@Param("userId") targetUserId: string,
+	) {
+		return this.messagesService.getConversationWithUser(
+			req.user.sub,
+			targetUserId,
+		);
+	}
+
 	@Get("conversations/:id")
 	getMessages(
 		@Req() req: AuthenticatedRequest,