feat(admissions): surface multi-year admissions trend on school detail

The school detail page only showed the latest admissions year. We store
every year, which is more decision-relevant for parents (the trend and its
consistency matter more than a single noisy year).

Backend now returns the full admissions_history (oldest first) alongside the
existing latest-year object. The primary SchoolDetailView gains a header
toggle ("This year | N-year trend") that swaps the Q&A for an SVG sparkline
of the first-choice offer rate. The toggle only appears when >=2 years carry
an offer rate; otherwise it falls back to the single-year card. Both views
share one CSS-grid cell so switching causes no layout shift.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

nextjs-app/app/school/[slug]/page.tsx

@@ -133,7 +133,7 @@ export default async function SchoolPage({ params }: SchoolPageProps) {
     notFound();
   }
 
-  const { school_info, yearly_data, absence_data, ofsted, parent_view, census, admissions, sen_detail, phonics, deprivation, finance } = data;
+  const { school_info, yearly_data, absence_data, ofsted, parent_view, census, admissions, admissions_history, sen_detail, phonics, deprivation, finance } = data;
 
   // Redirect bare URN to canonical slug URL
   const canonicalSlug = schoolUrl(urn, school_info.school_name).replace('/school/', '');
@@ -206,6 +206,7 @@ export default async function SchoolPage({ params }: SchoolPageProps) {
           parentView={parent_view ?? null}
           census={census ?? null}
           admissions={admissions ?? null}
+          admissionsHistory={admissions_history ?? []}
           senDetail={sen_detail ?? null}
           phonics={phonics ?? null}
           deprivation={deprivation ?? null}