feat: add AI-powered pack cover generation with 3 style options

Commit notes

- Add ai-cover-generation service using FAL.ai for image generation - Implement 3 cover styles: abstract, photographic, illustrated - Add routes for generating, refining, and applying AI covers - Create AICoverGenerator component with iterative refinement UI - Integrate AI cover generator into PackEditDrawer - Support custom prompts for influencing generation - Allow users to regenerate individual styles - Add FAL_KEY to config schema for FAL.ai integration

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