feat: Markdown 편집기 + PDF 변환 파이프라인 + 뷰어 포맷 분기

- Markdown split editor: textarea + marked preview, Ctrl+S 저장
- PUT /api/documents/{id}/content: 원본 파일 저장 + extracted_text 갱신
- GET /api/documents/{id}/preview: PDF 미리보기 캐시 서빙
- preview_worker: LibreOffice headless → PDF 변환 (timeout 60s, retry 1회)
- queue_consumer: preview stage 추가 (embed 후 자동 트리거)
- DocumentViewer: 포맷별 분기 (markdown/pdf/preview-pdf/image/text/cad)
- 오피스/CAD 문서: 새 탭 편집 버튼
- Dockerfile: LibreOffice headless 설치
- migration 005: preview_status, preview_hash, preview_at 컬럼

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

app/Dockerfile

@@ -2,6 +2,11 @@ FROM python:3.11-slim
 
 WORKDIR /app
 
+# LibreOffice headless (PDF 변환용)
+RUN apt-get update && \
+    apt-get install -y --no-install-recommends libreoffice-core libreoffice-calc libreoffice-writer libreoffice-impress && \
+    apt-get clean && rm -rf /var/lib/apt/lists/*
+
 COPY requirements.txt .
 RUN pip install --no-cache-dir -r requirements.txt