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/components/SecondarySchoolDetailView.tsx

@@ -618,6 +618,7 @@ export function SecondarySchoolDetailView({
                   schoolName={schoolInfo.school_name}
                   isSecondary={true}
                   nationalAtt8Avg={heroAtt8Nat}
+                  nationalByYear={nationalAvg?.by_year}
                 />
               </div>
             </>