fix(backtest): roll back the portfolio-sim DB failures too

The first pass guarded the replay loop but not the portfolio-simulation block,
which re-fetches price columns and loads the benchmark and the live exit policy
from the same session much later. A failure in any of those swallows the
exception without clearing the transaction — the identical failure mode, with
the identical symptom: the report write is the first unguarded statement and
takes the blame.

The outer handler is the backstop for the price_columns loop, which has no
handler of its own; rolling back a session an inner handler already cleared is
a no-op.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

tests/unit/test_backtest_service.py

@@ -1656,3 +1656,35 @@ async def test_run_backtest_rolls_back_a_failed_ticker_fetch(session, monkeypatc
     # the surviving ticker is still replayed after the rollback
     assert report["tickers"] == 2
     assert report["candidates"] >= 1
+
+
+async def test_run_backtest_rolls_back_a_failed_portfolio_sim_load(session, monkeypatch):
+    """The portfolio-sim block loads the benchmark and the live exit policy from
+    the same session, well after the replay loop. A failure there poisons the
+    transaction exactly as one in the loop does, and the report write pays for it.
+    """
+    await _seed_oscillating_ticker(session, "OSC")
+
+    rolled_back: list[str] = []
+    called: list[str] = []
+
+    async def failing_exit_policy(db):
+        called.append("x")
+        raise RuntimeError("simulated exit-policy read failure")
+
+    real_rollback = session.rollback
+
+    async def tracking_rollback():
+        rolled_back.append("x")
+        await real_rollback()
+
+    monkeypatch.setattr(
+        "app.services.paper_trade_service.get_exit_policy", failing_exit_policy
+    )
+    monkeypatch.setattr(session, "rollback", tracking_rollback)
+
+    report = await bt.run_backtest(session)
+
+    assert called, "the portfolio-sim block never ran; test proves nothing"
+    assert rolled_back, "a failed portfolio-sim load left the session un-rolled-back"
+    assert report["tickers"] == 1