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/data_loader.py

@@ -477,10 +477,9 @@ def get_supplementary_data(db: Session, urn: int) -> dict:
         else None
     )
 
-    # Admissions (latest year)
-    a = safe_query(FactAdmissions, "urn", "year")
-    result["admissions"] = (
-        {
+    # Admissions — all years, oldest first (for the multi-year trend view).
+    def _admissions_row(a):
+        return {
             "year": a.year,
             "school_phase": a.school_phase,
             "places_offered": a.places_offered,
@@ -491,9 +490,25 @@ def get_supplementary_data(db: Session, urn: int) -> dict:
             "oversubscription_ratio": a.oversubscription_ratio,
             "oversubscribed": a.oversubscribed,
         }
-        if a
-        else None
-    )
+
+    try:
+        admissions_rows = (
+            db.query(FactAdmissions)
+            .filter(FactAdmissions.urn == urn)
+            .order_by(FactAdmissions.year.asc())
+            .all()
+        )
+    except Exception as e:
+        import logging
+        logging.getLogger(__name__).error("admissions history query failed: %s", e)
+        db.rollback()
+        admissions_rows = []
+
+    history = [_admissions_row(a) for a in admissions_rows]
+    result["admissions_history"] = history
+    # Keep the single latest-year object for backwards-compatible consumers
+    # (hero chips, etc.).
+    result["admissions"] = history[-1] if history else None
 
     # SEN detail — not available in current marts
     result["sen_detail"] = None