Disallow Search Alert updates from changing the search type

[albertisfu]

In #4918 I found that it can be problematic if a user updates one of their Search Alerts via the API and changes the query to a new search type.

For instance, if the update goes from q=test&type=oa → q=test1&type=r.

In this case, the alert will be indexed into the RECAP percolator, but it will remain in the Oral Arguments percolator. This can lead to orphaned alerts in the index that will continue to be triggered.

The solution is to disable the ability to change the search type in an Alert. We can enforce this validation at the model level, ensuring that both the API and the frontend benefit from it. Although it is unlikely to happen in the frontend since the search type is a hidden field in the Alert form, adding the validation at the model level ensures this issue is avoided even if a user modifies the HTML before submitting the form.

[mlissner]

What's your size estimate for this, Alberto?

[albertisfu]

I think this is "small" size.

[mlissner]

Great. Let's see if this becomes a problem, and we can revisit it if so.