Rename ambiguous loop var l to lb in portfolio monitor

Fixes ruff E741 in the lookbacks comprehension of _portfolio_monitor,
which failed the CI lint step and blocked the deploy.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

app/services/backtest_service.py

@@ -1865,8 +1865,8 @@ def _portfolio_monitor(
             for s in PORTFOLIO_MONITOR_STRATEGIES
         ],
         "lookbacks": [
-            {"lookback": l["lookback"], "label": l["label"]}
-            for l in PORTFOLIO_MONITOR_LOOKBACKS
+            {"lookback": lb["lookback"], "label": lb["label"]}
+            for lb in PORTFOLIO_MONITOR_LOOKBACKS
         ],
         "runs": rows,
         "note": (