Add Streamlit web UI and Docker support

- app.py: Streamlit-based web UI for file-to-markdown conversion
- Dockerfile.ui: Docker image for UI (ffmpeg, ExifTool, port 8501)
- .dockerignore: whitelist app.py for Docker build
- CLAUDE.md: project instructions for Claude Code

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Dockerfile.ui

@@ -0,0 +1,34 @@
+FROM python:3.13-slim-bullseye
+
+ENV DEBIAN_FRONTEND=noninteractive
+ENV EXIFTOOL_PATH=/usr/local/bin/exiftool
+ENV FFMPEG_PATH=/usr/bin/ffmpeg
+
+RUN apt-get update && apt-get install -y --no-install-recommends \
+    ffmpeg \
+    curl \
+    perl \
+    make \
+    && rm -rf /var/lib/apt/lists/* \
+    && curl -fsSL https://exiftool.org/Image-ExifTool-13.55.tar.gz -o /tmp/exiftool.tar.gz \
+    && tar -xzf /tmp/exiftool.tar.gz -C /tmp \
+    && cd /tmp/Image-ExifTool-13.55 \
+    && perl Makefile.PL && make install \
+    && rm -rf /tmp/exiftool.tar.gz /tmp/Image-ExifTool-13.55
+
+WORKDIR /app
+COPY packages/ /app/packages/
+COPY app.py /app/app.py
+
+RUN pip --no-cache-dir install \
+    /app/packages/markitdown[all] \
+    streamlit
+
+EXPOSE 8501
+
+HEALTHCHECK CMD curl -f http://localhost:8501/_stcore/health || exit 1
+
+ENTRYPOINT ["streamlit", "run", "app.py", \
+    "--server.port=8501", \
+    "--server.address=0.0.0.0", \
+    "--server.headless=true"]