feat: adopt Phase 3 gate and paper-trade exit policy

Production strategy change based on the July 2026 backtest: paper trades now default to a 30-trading-day hold with the initial stop (classic momentum hold-and-rerank), while target and trailing exits remain available in Admin. The exit policy API/UI now carries hold_days and close_reason can be 'time'.

The activation confidence floor default is now 0/off because the gate ablation showed it added no per-trade edge while filtering out usable setups. Migration 015 clears stored activation_min_confidence and paper_exit_mode so the new defaults take effect; this intentionally resets Track Record comparability from this deploy.

Verification: 451 backend tests pass, ruff check app/ clean, frontend npm run build clean.

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

frontend/src/components/admin/ExitPolicySettings.tsx

@@ -6,13 +6,15 @@ import { SkeletonCard } from '../ui/Skeleton';
 export function ExitPolicySettings() {
   const { data, isLoading } = useExitPolicy();
   const update = useUpdateExitPolicy();
-  const [mode, setMode] = useState<ExitPolicy['mode']>('trailing');
+  const [mode, setMode] = useState<ExitPolicy['mode']>('time');
   const [pct, setPct] = useState(12);
+  const [holdDays, setHoldDays] = useState(30);
 
   useEffect(() => {
     if (data) {
       setMode(data.mode);
       setPct(data.trailing_pct);
+      setHoldDays(data.hold_days ?? 30);
     }
   }, [data]);
 
@@ -24,12 +26,14 @@ export function ExitPolicySettings() {
         <h3 className="text-sm font-semibold text-gray-200">Paper-Trade Exit</h3>
         <p className="mt-1 text-xs text-gray-500">
           How open paper trades auto-close (in the nightly/intraday outcome job).{' '}
-          <span className="text-gray-300">Trailing</span> rides a trailing stop — the backtest's best exit,
-          it lets winners run; <span className="text-gray-300">Target / stop</span> closes at the setup's
-          target or stop. The setup's initial stop is always the floor.
+          <span className="text-gray-300">Hold</span> keeps the initial stop and exits at the Nth trading
+          day's close — the backtest-validated exit (classic momentum: hold ~a month, re-rank);{' '}
+          <span className="text-gray-300">Trailing</span> rides a trailing stop;{' '}
+          <span className="text-gray-300">Target / stop</span> closes at the setup's target or stop.
+          The setup's initial stop is always the floor.
         </p>
       </div>
-      <div className="grid gap-4 md:grid-cols-2">
+      <div className="grid gap-4 md:grid-cols-3">
         <label className="block space-y-1">
           <span className="text-xs text-gray-400">Exit mode</span>
           <select
@@ -37,10 +41,25 @@ export function ExitPolicySettings() {
             onChange={(e) => setMode(e.target.value as ExitPolicy['mode'])}
             className="w-full input-glass px-3 py-2 text-sm"
           >
+            <option value="time">Hold N days + stop</option>
             <option value="trailing">Trailing stop</option>
             <option value="target">Target / stop</option>
           </select>
         </label>
+        <label className="block space-y-1">
+          <span className="text-xs text-gray-400">Hold (trading days)</span>
+          <input
+            type="number"
+            min={2}
+            max={250}
+            step={1}
+            value={holdDays}
+            onChange={(e) => setHoldDays(Number(e.target.value))}
+            disabled={mode !== 'time'}
+            className="w-full input-glass px-3 py-2 text-sm disabled:opacity-50"
+          />
+          <span className="text-[11px] text-gray-600">Backtest optimum: 30 (its evaluation horizon).</span>
+        </label>
         <label className="block space-y-1">
           <span className="text-xs text-gray-400">Trailing width (%)</span>
           <input
@@ -53,13 +72,13 @@ export function ExitPolicySettings() {
             disabled={mode !== 'trailing'}
             className="w-full input-glass px-3 py-2 text-sm disabled:opacity-50"
           />
-          <span className="text-[11px] text-gray-600">Give-back from the peak. Backtest sweet spot ~12–15%.</span>
+          <span className="text-[11px] text-gray-600">Give-back from the peak. ≥15% ≈ the hold exit.</span>
         </label>
       </div>
       <button
         className="btn-primary px-4 py-2 text-sm disabled:opacity-50"
         disabled={update.isPending}
-        onClick={() => update.mutate({ mode, trailing_pct: pct })}
+        onClick={() => update.mutate({ mode, trailing_pct: pct, hold_days: holdDays })}
       >
         {update.isPending ? 'Saving…' : 'Save Exit Policy'}
       </button>

frontend/src/lib/types.ts

@@ -211,14 +211,15 @@ export interface PaperTrade {
   benchmark_return_pct: number | null;
   alpha_pct: number | null;
   alpha_usd: number | null;
-  close_reason: 'trailing' | 'stop' | 'target' | 'manual' | null;
+  close_reason: 'time' | 'trailing' | 'stop' | 'target' | 'manual' | null;
   trailing_stop: number | null;
   trailing_distance_pct: number | null;
 }
 
 export interface ExitPolicy {
-  mode: 'trailing' | 'target';
+  mode: 'time' | 'trailing' | 'target';
   trailing_pct: number;
+  hold_days: number;
 }
 
 export interface BacktestBucket {