Fix filters API response structure

Backend returns filters directly at top level, not wrapped in 'filters' property.
Update FiltersResponse type and page components to match actual API response.

Fixes empty dropdowns for school types and local authorities.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

nextjs-app/lib/types.ts

@@ -182,9 +182,8 @@ export interface Filters {
   years: number[];
 }
 
-export interface FiltersResponse {
-  filters: Filters;
-}
+// Backend returns filters directly, not wrapped
+export type FiltersResponse = Filters;
 
 export interface MetricDefinition {
   key: string;