Add higher standard display and trend indicators to school cards

- Display RWM Higher % alongside RWM Expected % on school cards
- Add trend indicators (up/down/stable arrows) showing year-over-year change
- Backend calculates previous year's RWM for trend comparison
- Trend appears on cards and in school detail modal

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/app.py

@@ -238,11 +238,23 @@ async def get_schools(
     latest_year = df.groupby("urn")["year"].max().reset_index()
     df_latest = df.merge(latest_year, on=["urn", "year"])
 
+    # Calculate trend by comparing to previous year
+    # Get second-latest year for each school
+    df_sorted = df.sort_values(["urn", "year"], ascending=[True, False])
+    df_prev = df_sorted.groupby("urn").nth(1).reset_index()
+    if not df_prev.empty and "rwm_expected_pct" in df_prev.columns:
+        prev_rwm = df_prev[["urn", "rwm_expected_pct"]].rename(
+            columns={"rwm_expected_pct": "prev_rwm_expected_pct"}
+        )
+        df_latest = df_latest.merge(prev_rwm, on="urn", how="left")
+
     # Include key result metrics for display on cards
     location_cols = ["latitude", "longitude"]
     result_cols = [
         "year",
         "rwm_expected_pct",
+        "rwm_high_pct",
+        "prev_rwm_expected_pct",
         "reading_expected_pct",
         "writing_expected_pct",
         "maths_expected_pct",