feat: global material override on OutputType for x-ray/clay render modes

- Add `material_override` nullable column on OutputType (DB migration)
- When set, ALL product parts get rendered with this single material
- Override applies after alias resolution in render_order_line task
- Admin UI: dropdown in OutputType table to select a library material
- Display: amber badge showing active override material name

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/api/outputTypes.ts

@@ -17,6 +17,7 @@ export interface OutputType {
   pricing_tier_name: string | null
   price_per_item: number | null
   workflow_definition_id: string | null
+  material_override: string | null
   is_active: boolean
   created_at: string
   updated_at: string