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/app/domains/pipeline/tasks/render_order_line.py

@@ -198,9 +198,11 @@ def render_order_line_task(self, order_line_id: str):
                 from app.services.material_service import resolve_material_map
                 material_map = resolve_material_map(material_map)
 
-            # Apply global material override from OutputType (e.g. x-ray mode)
-            if line.output_type and line.output_type.material_override:
-                override_mat = line.output_type.material_override
+            # Apply material override: per-line override takes priority over output type override
+            _line_override = getattr(line, 'material_override', None)
+            _ot_override = line.output_type.material_override if line.output_type else None
+            override_mat = _line_override or _ot_override
+            if override_mat:
                 # Build override map from existing material_map keys or from parsed STEP parts
                 override_keys = set()
                 if material_map:
@@ -360,7 +362,7 @@ def render_order_line_task(self, order_line_id: str):
                         usd_path=usd_render_path,
                         focal_length_mm=focal_length_mm,
                         sensor_width_mm=sensor_width_mm,
-                        material_override=line.output_type.material_override if line.output_type else None,
+                        material_override=override_mat,
                     )
                     success = True
                     render_log = {
@@ -419,7 +421,7 @@ def render_order_line_task(self, order_line_id: str):
                     rotation_z=rotation_z,
                     focal_length_mm=focal_length_mm,
                     sensor_width_mm=sensor_width_mm,
-                    material_override=line.output_type.material_override if line.output_type else None,
+                    material_override=override_mat,
                     job_id=order_line_id,
                     order_line_id=order_line_id,
                     noise_threshold=noise_threshold,