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/lib/types.ts

@@ -315,6 +315,8 @@ export interface SchoolDetailsResponse {
   parent_view: OfstedParentView | null;
   census: SchoolCensus | null;
   admissions: SchoolAdmissions | null;
+  /** All available admissions years, oldest first. Drives the multi-year trend view. */
+  admissions_history: SchoolAdmissions[];
   sen_detail: SenDetail | null;
   phonics: Phonics | null;
   deprivation: SchoolDeprivation | null;