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)

docker-entrypoint.sh

@@ -0,0 +1,44 @@
+#!/bin/bash
+set -e
+
+echo "Starting DigiServer v2..."
+
+# Create necessary directories
+mkdir -p /app/instance
+mkdir -p /app/app/static/uploads
+
+# Initialize database if it doesn't exist
+if [ ! -f /app/instance/digiserver.db ]; then
+    echo "Initializing database..."
+    python -c "
+from app.app import create_app
+from app.extensions import db, bcrypt
+from app.models import User
+
+app = create_app()
+with app.app_context():
+    db.create_all()
+    
+    # Create admin user
+    admin = User.query.filter_by(username='admin').first()
+    if not admin:
+        hashed = bcrypt.generate_password_hash('admin123').decode('utf-8')
+        admin = User(username='admin', password=hashed, role='admin')
+        db.session.add(admin)
+        db.session.commit()
+        print('✅ Admin user created (admin/admin123)')
+    else:
+        print('✅ Admin user already exists')
+"
+    echo "Database initialized!"
+fi
+
+# Start the application
+echo "Starting Gunicorn..."
+exec gunicorn \
+    --bind 0.0.0.0:5000 \
+    --workers 4 \
+    --timeout 120 \
+    --access-logfile - \
+    --error-logfile - \
+    "app.app:create_app()"