Add Docker support and improve PDF conversion

Features:
- Dockerfile for containerized deployment
- Docker Compose configuration with health checks
- Automated database initialization via entrypoint script
- Quick start script for easy Docker deployment
- Comprehensive Docker deployment documentation (DOCKER.md)
- Complete README with installation and usage instructions
- .dockerignore for optimized image builds

Improvements:
- PDF conversion now preserves orientation (portrait/landscape)
- PDF rendering at 300 DPI for sharp quality
- Maintains aspect ratio during conversion
- Compact media library view with image thumbnails
- Better media preview with scrollable gallery

Docker Features:
- Multi-stage build for smaller images
- Non-root user for security
- Health checks for container monitoring
- Volume mounts for persistent data
- Production-ready Gunicorn configuration
- Support for Redis caching (optional)

Dockerfile

@@ -0,0 +1,50 @@
+# Use Python 3.13 slim image
+FROM python:3.13-slim
+
+# Set working directory
+WORKDIR /app
+
+# Install system dependencies
+RUN apt-get update && apt-get install -y \
+    poppler-utils \
+    libreoffice \
+    ffmpeg \
+    libmagic1 \
+    && rm -rf /var/lib/apt/lists/*
+
+# Copy requirements first for better caching
+COPY requirements.txt .
+
+# Install Python dependencies
+RUN pip install --no-cache-dir -r requirements.txt
+
+# Copy application code
+COPY . .
+
+# Copy and set permissions for entrypoint script
+COPY docker-entrypoint.sh /docker-entrypoint.sh
+RUN chmod +x /docker-entrypoint.sh
+
+# Create directories for uploads and database
+RUN mkdir -p app/static/uploads instance
+
+# Set environment variables
+ENV FLASK_APP=app.app:create_app
+ENV PYTHONUNBUFFERED=1
+ENV FLASK_ENV=production
+
+# Expose port
+EXPOSE 5000
+
+# Create a non-root user
+RUN useradd -m -u 1000 appuser && \
+    chown -R appuser:appuser /app /docker-entrypoint.sh
+
+USER appuser
+
+# Health check
+HEALTHCHECK --interval=30s --timeout=10s --start-period=40s --retries=3 \
+    CMD python -c "import urllib.request; urllib.request.urlopen('http://localhost:5000/').read()" || exit 1
+
+# Run the application via entrypoint
+ENTRYPOINT ["/docker-entrypoint.sh"]