feat: per-order-line material override — override materials for individual renders

- Add `material_override` nullable column on OrderLine (DB migration)
- Line override takes priority over OutputType override
- PATCH /orders/{id}/lines/{id} endpoint to update material_override
- Inline dropdown on each order line in the OrderDetail page
- Amber background when override is active
- Same output type, different material per line — no need to create a new output type

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

backend/alembic/versions/42b686e71c47_add_material_override_to_order_lines.py

@@ -0,0 +1,30 @@
+"""add material_override to order_lines
+
+Revision ID: 42b686e71c47
+Revises: cfcc7ad1e7d5
+Create Date: 2026-03-14 13:30:03.046793
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision: str = '42b686e71c47'
+down_revision: Union[str, None] = 'cfcc7ad1e7d5'
+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('order_lines', 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('order_lines', 'material_override')
+    # ### end Alembic commands ###