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>

backend/app.py

@@ -608,6 +608,7 @@ async def get_school_details(request: Request, urn: int):
         "parent_view": supplementary.get("parent_view"),
         "census": supplementary.get("census"),
         "admissions": supplementary.get("admissions"),
+        "admissions_history": supplementary.get("admissions_history") or [],
         "sen_detail": supplementary.get("sen_detail"),
         "phonics": supplementary.get("phonics"),
         "deprivation": supplementary.get("deprivation"),