Fix Gate/Binance memory regression and roll stop offset from avg.

Stop fetch_tickers fallback for volume rank and keep stale cache on failed refresh. Compute roll unified stop as merge-average plus offset percent instead of break-even.

Co-authored-by: Cursor <cursoragent@cursor.com>

strategy_db.py

@@ -154,6 +154,7 @@ def init_strategy_tables(conn) -> None:
         "ALTER TABLE order_monitors ADD COLUMN monitor_type TEXT",
         "ALTER TABLE order_monitors ADD COLUMN key_signal_type TEXT",
         "ALTER TABLE trend_pullback_plans ADD COLUMN leg_fill_prices_json TEXT",
+        "ALTER TABLE roll_legs ADD COLUMN stop_offset_pct REAL",
     ):
         try:
             conn.execute(ddl)