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>

backend/app/domains/rendering/schemas.py

@@ -17,6 +17,7 @@ class OutputTypeCreate(BaseModel):
     transparent_bg: bool = False
     pricing_tier_id: int | None = None
     cycles_device: str | None = None
+    material_override: str | None = None
 
 
 class OutputTypePatch(BaseModel):
@@ -34,6 +35,7 @@ class OutputTypePatch(BaseModel):
     pricing_tier_id: int | None = None
     cycles_device: str | None = None
     workflow_definition_id: uuid.UUID | None = None
+    material_override: str | None = None
 
 
 class OutputTypeOut(BaseModel):
@@ -54,6 +56,7 @@ class OutputTypeOut(BaseModel):
     price_per_item: float | None = None
     workflow_definition_id: uuid.UUID | None = None
     workflow_name: str | None = None
+    material_override: str | None = None
     is_active: bool
     created_at: datetime
     updated_at: datetime