feat: use DALL-E 2 for cheaper AI cover generation

Commit notes

- Switch from gpt-image-1 to dall-e-2 (~--.018 per image vs higher cost) - Use 512x512 resolution instead of 1024x1024 for placeholders - Faster generation and lower cost while still suitable for inspiration

Co-authored-by: armin.naimi <[email redacted]>