Add book covers

docs/cover-images.md

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+# Book Cover Images Setup
+
+HXBooks now supports book cover images using static file storage with nginx/caddy for production.
+
+## Architecture
+
+**Development**: Flask serves covers via `/media/covers/<filename>` route
+**Production**: nginx serves covers directly from filesystem for optimal performance
+
+## File Structure
+
+```
+/app/
+├── src/hxbooks/static/          # App assets (CSS, JS) -> served at /static/
+├── media/covers/                # Book covers -> served at /media/
+└── instance/                   # Database, config
+```
+
+## Production Deployment
+
+### Docker Setup
+```bash
+# Build and run with nginx + gunicorn
+docker-compose up --build
+
+# Or build manually
+docker build -f Dockerfile.production -t hxbooks .
+docker run -p 8080:80 -v ./media:/app/media -v ./instance:/app/instance hxbooks
+```
+
+### nginx Configuration
+- Static files: nginx serves `/static/` and `/media/` directly
+- Dynamic requests: proxied to gunicorn on port 8080
+- Proper caching headers for performance
+
+## Cover Image Features
+
+### CLI
+```bash
+# Import with cover download (default)
+hxbooks book import "9780123456789"
+
+# Skip cover download
+hxbooks book import "9780123456789" --no-cover
+```
+
+### Web Interface
+- Book cards display covers when available
+- Fallback to book emoji for books without covers
+- Google Books API integration provides cover URLs
+
+## Storage Details
+
+- **Images downloaded from**: Google Books API (thumbnail/smallThumbnail)
+- **Filename format**: `book_{id}.{extension}`
+- **Security**: HTTP URLs converted to HTTPS
+- **Fallback**: graceful handling if download fails
+- **Database**: `cover_image_path` field stores relative filename
+
+## Performance
+
+**nginx serves static files directly** (bypassing Python completely):
+- ✅ ~1ms response time for images
+- ✅ Proper browser caching
+- ✅ Gzip compression
+- ✅ No Python memory usage for images
+
+vs. database BLOB storage through Python:
+- ❌ ~50-200ms response time
+- ❌ Limited caching options  
+- ❌ High memory usage
+- ❌ Database file bloat