fix(rankings): expose selected metric under stable value key

The /api/rankings endpoint returned each row keyed by the metric's
column name (e.g. rwm_high_pct) but never under a generic `value`
field. The frontend RankingItem type and RankingsView both read
ranking.value, so every row rendered "—" for every metric — the
default rwm_expected_pct included.

Add `df["value"] = df[metric]` before JSON serialisation so the
frontend gets the value it has always expected. The raw metric
column is still in the row for any caller that wants it explicitly.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/app.py

@@ -818,7 +818,12 @@ async def get_rankings(
 
     # Return only relevant fields for rankings
     available_cols = [c for c in RANKING_COLUMNS if c in df.columns]
-    df = df[available_cols]
+    df = df[available_cols].copy()
+
+    # Surface the requested metric under a stable `value` key so the
+    # frontend doesn't need to know each metric's column name. The raw
+    # metric column is also kept in the row for callers that want it.
+    df["value"] = df[metric]
 
     return {
         "metric": metric,