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/api/routers/orders.py

@@ -96,6 +96,7 @@ def _build_line_out(line: OrderLine) -> OrderLineOut:
         render_log=line.render_log if hasattr(line, 'render_log') else None,
         render_started_at=line.render_started_at if hasattr(line, 'render_started_at') else None,
         render_completed_at=line.render_completed_at if hasattr(line, 'render_completed_at') else None,
+        material_override=getattr(line, 'material_override', None),
         notes=line.notes,
         created_at=line.created_at,
         updated_at=line.updated_at,
@@ -1096,6 +1097,42 @@ async def dispatch_single_line_render(
     return {"dispatched": True, "line_id": str(line.id)}
 
 
+class PatchLineBody(BaseModel):
+    material_override: str | None = None
+
+
+@router.patch("/{order_id}/lines/{line_id}")
+async def patch_order_line(
+    order_id: uuid.UUID,
+    line_id: uuid.UUID,
+    body: PatchLineBody,
+    user: User = Depends(require_admin_or_pm),
+    db: AsyncSession = Depends(get_db),
+):
+    """Update fields on an order line (admin/PM only)."""
+    result = await db.execute(select(Order).where(Order.id == order_id))
+    order = result.scalar_one_or_none()
+    if not order:
+        raise HTTPException(404, detail="Order not found")
+
+    line_result = await db.execute(
+        select(OrderLine).where(OrderLine.id == line_id, OrderLine.order_id == order_id)
+    )
+    line = line_result.scalar_one_or_none()
+    if not line:
+        raise HTTPException(404, detail="Order line not found")
+
+    data = body.model_dump(exclude_unset=True)
+    from sqlalchemy import update as sql_update
+    if data:
+        await db.execute(
+            sql_update(OrderLine).where(OrderLine.id == line.id).values(**data)
+        )
+        await db.commit()
+
+    return {"updated": True, "line_id": str(line.id)}
+
+
 class RejectLineBody(BaseModel):
     reason: str = ""