Upload, Replace, or Delete Experience Asset

**POST:** Upload a logo or cover image for a specific experience type within an experience. If experience_type is omitted and only one type exists (or only one selected), it will be inferred. **DELETE:** Remove a logo or cover image from a specific experience type. Validation rules: (1) asset_type must be 'logo' or 'cover'; (2) experience_type must either be provided or uniquely inferable; (3) experience_type must already exist on the experience (selected_experience_types).