feat: Presigned URL API 및 OpenTelemetry trace 추가

- Presigned URL API 추가 (MinIO/S3 private 버킷 지원)
- OpenTelemetry trace 설정 추가 (DEBUG=False 시 활성화)
- requirements.txt에 opentelemetry 패키지 추가

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

blog/urls.py

@@ -6,7 +6,8 @@ from .views import (
     PostListView, PostListCreateView, PostDetailView,
     CommentViewSet, TagListView,
     AttachmentListCreateView, AttachmentDeleteView,
-    TempAttachmentUploadView, TempAttachmentDeleteView
+    TempAttachmentUploadView, TempAttachmentDeleteView,
+    PresignedUrlView
 )
 
 # 댓글 라우터
@@ -36,4 +37,8 @@ urlpatterns = [
          TempAttachmentUploadView.as_view(), name='temp-attachment-upload'),
     path('attachments/<int:pk>/',
          TempAttachmentDeleteView.as_view(), name='temp-attachment-delete'),
+
+    # Presigned URL (MinIO/S3)
+    path('presigned/<path:object_key>/',
+         PresignedUrlView.as_view(), name='presigned-url'),
 ]