feat: batch material override — apply to all lines in an order at once

- POST /orders/{id}/batch-material-override endpoint
- Dropdown above the lines table: "Apply to all lines…"
- Options: clear all overrides, or select a library material
- Updates all order lines in one request

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

backend/app/api/routers/orders.py

@@ -1097,6 +1097,32 @@ async def dispatch_single_line_render(
     return {"dispatched": True, "line_id": str(line.id)}
 
 
+class BatchMaterialOverrideBody(BaseModel):
+    material_override: str | None = None
+
+
+@router.post("/{order_id}/batch-material-override")
+async def batch_material_override(
+    order_id: uuid.UUID,
+    body: BatchMaterialOverrideBody,
+    user: User = Depends(require_admin_or_pm),
+    db: AsyncSession = Depends(get_db),
+):
+    """Set material_override on ALL lines of an order at once."""
+    result = await db.execute(select(Order).where(Order.id == order_id))
+    if not result.scalar_one_or_none():
+        raise HTTPException(404, detail="Order not found")
+
+    from sqlalchemy import update as sql_update
+    res = await db.execute(
+        sql_update(OrderLine)
+        .where(OrderLine.order_id == order_id)
+        .values(material_override=body.material_override)
+    )
+    await db.commit()
+    return {"updated": res.rowcount, "material_override": body.material_override}
+
+
 class PatchLineBody(BaseModel):
     material_override: str | None = None