feat: national average reference line now tracks per year on history chart

Previously the dashed reference line was a flat horizontal at the latest
year's national average across all historical data, implying the national
figure was constant. Now the backend returns per-year averages in `by_year`
and the chart maps each data year to its own national average, so the
reference line correctly reflects how the national picture changed over time
(including COVID recovery dip/recovery).

- backend: /api/national-averages now includes `by_year` list alongside
  existing `year`/`primary`/`secondary` latest-year snapshot
- types: NationalAverages extended with `by_year: NationalAveragesYear[]`
- PerformanceChart: accepts `nationalByYear` prop; builds per-year series
  aligned to school data years, falling back to scalar prop if absent
- SchoolDetailView + SecondarySchoolDetailView: pass `nationalAvg.by_year`

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

nextjs-app/lib/types.ts

@@ -351,12 +351,17 @@ export interface Filters {
   admissions_policies: string[];
 }
 
-export interface NationalAverages {
+export interface NationalAveragesYear {
   year: number;
   primary: Record<string, number>;
   secondary: Record<string, number>;
 }
 
+export interface NationalAverages extends NationalAveragesYear {
+  /** Per-year averages for every year in the dataset, used for chart reference lines. */
+  by_year: NationalAveragesYear[];
+}
+
 export interface LAaveragesResponse {
   year: number;
   secondary: {