Set param to ecf_id for new API routes (#1322)

There's a source of potential confusion where we're dealing with ecf_id
and referring to it as id. I think it's clearer if we override param so
it's clear exactly what we're referring to.

app/controllers/api/v3/statements_controller.rb

@@ -11,7 +11,7 @@ def index
       end
 
       def show
-        render json: to_json(statements_query.statement(ecf_id: statement_params[:id]))
+        render json: to_json(statements_query.statement(ecf_id: statement_params[:ecf_id]))
       end
 
     private
@@ -25,7 +25,7 @@ def statements_query
       end
 
       def statement_params
-        params.permit(:id, filter: %i[cohort updated_since])
+        params.permit(:ecf_id, filter: %i[cohort updated_since])
       end
 
       def cohort_start_years

config/routes.rb

@@ -177,7 +177,7 @@
         put :void, path: "void"
       end
 
-      resources :statements, only: %i[index show]
+      resources :statements, only: %i[index show], param: :ecf_id
     end
   end