Fix backtest benchmark read after optional table probe

app/services/backtest_service.py

@@ -937,13 +937,16 @@ async def _load_research_rank_only_symbols(db: AsyncSession) -> set[str]:
     """Symbols that feed signal IC only (no GTL/candidate replay).
 
     Optional side table ``research_rank_only`` on research snapshots. Missing
-    table → empty set (production path unchanged).
+    table → empty set (production path unchanged). Probe within a savepoint:
+    PostgreSQL aborts the current transaction after a missing-table error, so
+    catching that error alone would break the subsequent benchmark read.
     """
     from sqlalchemy import text
 
     try:
-        result = await db.execute(text("SELECT symbol FROM research_rank_only"))
-        return {str(row[0]).upper() for row in result.fetchall() if row[0]}
+        async with db.begin_nested():
+            result = await db.execute(text("SELECT symbol FROM research_rank_only"))
+            return {str(row[0]).upper() for row in result.fetchall() if row[0]}
     except Exception:
         return set()