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>

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 "")