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/models.py

@@ -44,6 +44,8 @@ class OutputType(Base):
         DateTime, default=datetime.utcnow, onupdate=datetime.utcnow, nullable=False
     )
 
+    material_override: Mapped[str | None] = mapped_column(String(200), nullable=True, default=None)
+
     workflow_definition_id: Mapped[uuid.UUID | None] = mapped_column(
         UUID(as_uuid=True), ForeignKey("workflow_definitions.id", ondelete="SET NULL"), nullable=True
     )