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>

frontend/src/components/signals/BacktestPanel.tsx

@@ -184,19 +184,19 @@ export function BacktestPanel() {
             {report.sweep && report.sweep.length > 0 && (
               <div>
                 <p className="mb-2 text-xs font-medium uppercase tracking-widest text-gray-500">
-                  Min expected-value sweep
+                  Momentum-percentile sweep
                 </p>
                 <p className="mb-2 text-[11px] text-gray-500">
-                  How many setups qualify — and how they perform — at each expected-value gate (other
-                  gate conditions held fixed). EV is in R: 0.15 means +0.15× your risk per trade on
-                  average. Lower = more trades, watch that expectancy holds. Your current setting is
+                  How many setups qualify — and how they perform — at each momentum-rank cutoff (floors
+                  held fixed). 80 = only the top 20% of the universe by 12-1 momentum each week; 0 =
+                  floors only. Lower = more trades, watch that expectancy holds. Your current setting is
                   highlighted; set it in Admin → Settings → Activation.
                 </p>
                 <div className="glass overflow-x-auto">
                   <table className="w-full text-sm">
                     <thead>
                       <tr className="border-b border-white/[0.06] text-left text-xs uppercase tracking-wider text-gray-500">
-                        <th className="px-4 py-2.5">Min EV (R)</th>
+                        <th className="px-4 py-2.5">Min momentum %ile</th>
                         <th className="px-4 py-2.5 text-right">Qualified</th>
                         <th className="px-4 py-2.5 text-right">Wins</th>
                         <th className="px-4 py-2.5 text-right">Losses</th>
@@ -207,12 +207,12 @@ export function BacktestPanel() {
                     </thead>
                     <tbody>
                       {report.sweep.map((row) => {
-                        const current = Math.abs(row.min_expected_value - report.min_expected_value) < 0.001;
+                        const current = Math.abs(row.min_momentum_percentile - report.min_momentum_percentile) < 0.001;
                         return (
-                          <tr key={row.min_expected_value} className={`border-b border-white/[0.04] ${current ? 'bg-blue-400/10' : ''}`}>
+                          <tr key={row.min_momentum_percentile} className={`border-b border-white/[0.04] ${current ? 'bg-blue-400/10' : ''}`}>
                             <td className="num px-4 py-2.5 text-gray-200">
                               {current && <span className="mr-1 text-blue-300">★</span>}
-                              {row.min_expected_value.toFixed(2)}
+                              {row.min_momentum_percentile.toFixed(0)}
                             </td>
                             <td className="num px-4 py-2.5 text-right text-gray-200">{row.total}</td>
                             <td className="num px-4 py-2.5 text-right text-emerald-400">{row.wins}</td>