Update mascot settings

Updates mascot export settings. image_exports configures future PNG/WebP derivatives of original, transparent and sticker images; it does not backfill existing images. animation_sizes preserves existing behavior: changed settings may queue exports for existing animations as well as future outputs. Use POST /v1/mascots/{id}/exports for explicit image/video backfill, or the legacy size-variants endpoint for video-only repair. Returns settings_applied, size_variant_jobs and sizes_enabled. No credit cost.