Upload was failing because:
1. Missing 'apikey' header on PUT request to Supabase Storage
2. Public URL needs ?apikey= query param for this Supabase instance

Fixed:
- Added 'apikey: SERVICE_KEY' header to upload curl request
- Public URL now includes anon key: .../branding/slot.ext?apikey=ANON_KEY
- Browser can load the image directly from this URL
- Survives deploys (stored in Supabase Storage, URL in platform_assets)
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>