fix: batched infographic generation (60s timeout)

Batched infographic generation across cron cycles to fit within Vercel Hobby 60s timeout.\n\nstepResearchComplete → instant setup (0 Imagen calls)\nstepInfographicsGenerating → BATCH_SIZE=5 prompts per cycle (~40s)\nfinalizeInfographics → writes to root arrays + scene-level URLs\n\n18 prompts = 4 cycles × 5 min = 20 min total. Scales to any size.

Author: codercatdev