replace EV activation gate with cross-sectional 12-1 momentum ranking

The 5-year backtest confirmed the EV gate adds negative value (high threshold =
worst expectancy) and that 12-1 month momentum is the one price signal with a
plausible, right-signed cross-sectional IC (~0.05). So "qualified" now means:
clears the R:R + confidence floors AND the ticker ranks in the top
`min_momentum_percentile` of the universe by 12-1 momentum that week.

- qualification.py: drop expected_value_r / the EV gate; add a momentum-percentile
  gate (duck-typed `momentum_percentile`, only enforced when attached + threshold
  set, else defers to floors). Mirrored in frontend qualification.ts.
- activation config/schema: min_expected_value -> min_momentum_percentile
  (default 80 = top quintile). ActivationSettings, DashboardPage (ranks/【shows】
  momentum instead of EV), and the BacktestPanel sweep follow.
- backtest: rank each ISO week's universe by 12-1 momentum, assign a percentile,
  and qualify the top slice; the sweep now sweeps the percentile cutoff.

Also offload the backtest's per-ticker compute to a worker thread so the heavy
~5y run no longer blocks the API event loop (the "backend offline" flicker).

Production setups don't carry momentum_percentile yet — wiring the scanner to
attach it (a universe momentum-rank step) is the next step; until then the live
gate defers to floors while the backtest measures the momentum selection. 330
backend tests pass; frontend build clean.

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

tests/unit/test_activation_settings.py

@@ -25,7 +25,7 @@ class TestActivationConfig:
     async def test_defaults_when_unset(self, session: AsyncSession):
         config = await get_activation_config(session)
         assert config == {
-            "min_expected_value": 0.15,
+            "min_momentum_percentile": 80.0,
             "min_rr": 1.2,
             "min_confidence": 55.0,
             "min_target_probability": 0.0,
@@ -35,13 +35,13 @@ class TestActivationConfig:
 
     async def test_update_and_read_back(self, session: AsyncSession):
         updated = await update_activation_config(
-            session, {"min_expected_value": 0.25, "min_confidence": 60.0}
+            session, {"min_momentum_percentile": 70.0, "min_confidence": 60.0}
         )
-        assert updated["min_expected_value"] == 0.25
+        assert updated["min_momentum_percentile"] == 70.0
         assert updated["min_confidence"] == 60.0
 
         config = await get_activation_config(session)
-        assert config["min_expected_value"] == 0.25
+        assert config["min_momentum_percentile"] == 70.0
         assert config["min_confidence"] == 60.0
 
     async def test_partial_update_keeps_other_value(self, session: AsyncSession):
@@ -50,9 +50,9 @@ class TestActivationConfig:
         assert config["min_rr"] == 1.2  # default untouched
         assert config["min_confidence"] == 80.0
 
-    async def test_rejects_out_of_range_expected_value(self, session: AsyncSession):
+    async def test_rejects_out_of_range_momentum_percentile(self, session: AsyncSession):
         with pytest.raises(ValidationError):
-            await update_activation_config(session, {"min_expected_value": 50.0})
+            await update_activation_config(session, {"min_momentum_percentile": 150.0})
 
     async def test_conviction_flags_round_trip(self, session: AsyncSession):
         await update_activation_config(