fix: always GROUP BY me.id in market_events list query

The LEFT JOIN on instrument_impacts (added for the evaluated subquery)
generates N rows per event when the event has N impacts. GROUP BY was
only applied when the instrument filter was active — now it's always on.
Fixes: total=2 but list shows 6+ duplicates.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/routers/market_events.py

@@ -168,8 +168,9 @@ def list_events(
         sort_col = "me.impact_score"
     order_sql = f"ORDER BY {sort_col} {'DESC' if sort_dir == 'desc' else 'ASC'}"
 
-    # When joining with instrument_impacts an event can appear multiple times — use GROUP BY
-    group_sql = "GROUP BY me.id" if instrument else ""
+    # Always GROUP BY — the LEFT JOIN on instrument_impacts creates duplicate rows
+    # when an event has multiple impacts (one row per impact without GROUP BY)
+    group_sql = "GROUP BY me.id"
 
     # Extra columns: evaluated flag + optional instrument score
     evaluated_col = (