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/alembic/versions/cfcc7ad1e7d5_add_material_override_to_output_types.py

@@ -0,0 +1,30 @@
+"""add material_override to output_types
+
+Revision ID: cfcc7ad1e7d5
+Revises: 4c15abf3cf40
+Create Date: 2026-03-14 12:12:03.706587
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision: str = 'cfcc7ad1e7d5'
+down_revision: Union[str, None] = '4c15abf3cf40'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    # ### commands auto generated by Alembic - please adjust! ###
+    op.add_column('output_types', sa.Column('material_override', sa.String(length=200), nullable=True))
+    # ### end Alembic commands ###
+
+
+def downgrade() -> None:
+    # ### commands auto generated by Alembic - please adjust! ###
+    op.drop_column('output_types', 'material_override')
+    # ### end Alembic commands ###