feat: implement direct file upload functionality with S3 integration

apps/backend/src/lib/s3.ts

@@ -37,7 +37,7 @@ async function uploadToS3(fileName: string, mediaUrl: string, maxRetry = 3) {
       }
 
       const buffer = await response.arrayBuffer();
-      
+
       // 2. Bun S3 Client만 사용하여 쓰기
       // client.write는 내부적으로 효율적인 스트리밍/버퍼 처리를 수행합니다.
       await client.write(fileName, buffer, {
@@ -67,4 +67,11 @@ async function uploadToS3(fileName: string, mediaUrl: string, maxRetry = 3) {
   throw lastError;
 }
 
-export { makeS3FileName, uploadToS3, client as s3Client };
+async function writeToS3(fileName: string, data: ArrayBuffer | Blob, contentType: string) {
+  await warmupS3();
+  await client.write(fileName, data, {
+    type: contentType,
+  });
+}
+
+export { makeS3FileName, uploadToS3, writeToS3, client as s3Client };