feat: switch to Fal Flux Schnell for AI cover generation

Commit notes

- Replace DALL-E 2 with Fal Flux Schnell (~$0.003 per image) - Much faster generation (1-2 seconds vs 10+ seconds) - Uses FAL_KEY environment variable (already configured) - Returns URLs that work with existing apply endpoint - 4 inference steps optimized for Schnell model

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