fix(backtest): rebuild the recommendation on read, not only on run

Every fix so far only applied to reports generated after deploy. The cached
report is served verbatim, so it keeps the recommendation the OLD build stored —
quoting the legacy policy book, naming a rejected exit as "recommended", and
carrying no basis_lookback, which let the lookback selector default to 3y and
put 3-year tiles beside an all-history recommendation with no divergence notice.
Exactly the contradiction the last three commits set out to remove, silently
present on the first page load after deploy and until the next scheduled run
overwrote it.

The recommendation is a pure function of the numbers already in the report — its
own note says it is derived from them on every run — so it is now re-derived on
read. A corrected recommendation appears immediately instead of after the next
backtest. On failure it is dropped rather than falling back to the stored one,
which is the stale derivation this replaces.

The test drives the real shape: an old-build report with a legacy recommendation
written straight to the settings row, read back through get_backtest_report.

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

app/services/backtest_service.py

@@ -4436,11 +4436,38 @@ async def run_and_store(
 
 
 async def get_backtest_report(db: AsyncSession) -> dict | None:
-    """Return the last cached backtest report, or None if never run."""
+    """Return the last cached backtest report, or None if never run.
+
+    The recommendation is **re-derived from the cached report** rather than
+    served as stored. It is a pure function of the numbers already in the
+    report — the payload's own note says it is derived from them on every run —
+    so recomputing costs nothing and keeps one class of bug out:
+
+    A report cached by an older build carries that build's recommendation. After
+    a change to how the recommendation is sourced, the page would keep showing
+    the old one — quoting the legacy policy book, naming a rejected exit as
+    "recommended", and omitting ``basis_lookback``, which in turn let the
+    lookback selector default somewhere else. The result was the exact
+    tiles-disagree-with-recommendation contradiction this rebuild exists to
+    prevent, silently, until the next scheduled run happened to overwrite it.
+
+    Re-deriving means a corrected recommendation appears on the first page load
+    after deploy instead of after the next backtest.
+    """
     setting = await settings_store.get_setting(db, KEY_REPORT)
     if setting is None:
         return None
     try:
-        return json.loads(setting.value)
+        report = json.loads(setting.value)
     except (TypeError, ValueError):
         return None
+    if not isinstance(report, dict):
+        return None
+    try:
+        report["recommendation"] = _build_recommendation(report)
+    except Exception:
+        # Fail closed: drop it rather than fall back to the stored one, which is
+        # precisely the stale derivation this rebuild is here to replace.
+        logger.exception("Could not rebuild the backtest recommendation; omitting it")
+        report.pop("recommendation", None)
+    return report

tests/unit/test_backtest_service.py

@@ -2,6 +2,7 @@
 
 from __future__ import annotations
 
+import json
 import math
 from datetime import date, timedelta
 from types import SimpleNamespace
@@ -1862,3 +1863,58 @@ def test_build_recommendation_states_no_baseline_without_a_production_row():
     assert "benchmark" not in topics
     assert "exit" not in topics
 
+
+async def test_cached_report_recommendation_is_rebuilt_on_read(session):
+    """A report cached by an older build carries that build's recommendation.
+
+    Served verbatim, the page would show the legacy wording and no
+    basis_lookback — which let the lookback selector default elsewhere, putting
+    3y tiles beside an all-history recommendation with no warning. This is the
+    shape of the report sitting in production right now.
+    """
+    from app.services.admin_service import update_setting
+
+    stale = {
+        "generated_at": "2026-08-12T05:00:00+00:00",
+        "tickers": 512, "candidates": 100, "qualified": 10,
+        "params": {"horizon_days": 30},
+        "overall_qualified": {"net_avg_r": 0.13, "net_avg_r_ex_top5": 0.20},
+        "portfolio_sim": {"policies": [
+            {"policy": "hold", "cagr_pct": 31.9, "total_return_pct": 175.0,
+             "spy_return_pct": 101.9, "max_drawdown_pct": 23.7},
+        ]},
+        "portfolio_monitor": {
+            "production_strategy": "prod",
+            "runs": [{
+                "strategy": "prod", "lookback": "all", "lookback_label": "All history",
+                "cagr_pct": 40.0, "sharpe": 1.72, "max_drawdown_pct": 17.7,
+                "total_return_pct": 297.8, "spy_return_pct": 101.9,
+            }],
+        },
+        # What the old build stored: sourced from the policy book, and naming an
+        # exit the production book replaced.
+        "recommendation": {
+            "headline": "Trade the qualified list long-only; hold 30 trading days.",
+            "items": [
+                {"topic": "benchmark", "text": "Book vs SPY: beats buy-and-hold by "
+                                               "+73.1 points (+175.0% vs +101.9%)."},
+                {"topic": "robustness", "text": "Robustness: expectancy survives removing "
+                                                "the top 5% of winners (+0.20R net/trade "
+                                                "under the recommended 30d hold)."},
+            ],
+            "note": "stale",
+        },
+    }
+    await update_setting(session, bt.KEY_REPORT, json.dumps(stale))
+
+    report = await bt.get_backtest_report(session)
+    assert report is not None
+    rec = report["recommendation"]
+
+    # Rebuilt: the basis is published, so the page cannot default elsewhere.
+    assert rec["basis_lookback"] == "all"
+    texts = " | ".join(i["text"] for i in rec["items"])
+    # ...and it quotes the production book, not the policy sim it used to.
+    assert "+297.8%" in texts and "175.0" not in texts
+    assert "recommended 30d hold" not in texts
+    assert "Production baseline" in (rec["headline"] or "")