feat: trailing-stop auto-exit for paper trades + close/digest alerts

Applies the backtest-validated trailing stop to live paper trading, and surfaces
it transparently.

Exit (A):
- New paper-trade exit policy (paper_exit_mode=trailing, paper_trailing_pct=12),
  tunable in Admin → Paper-Trade Exit. resolve_open_trades runs a trailing stop
  (initial stop as floor, ratchets up from the peak; target ignored — the
  validated rule) and records close_reason (trailing|stop|target|manual; +migration
  013).
- list_trades enriches open trades with the live trailing-stop level + distance %.
  Open Trades panel shows the active tactic and a Trail Stop column.

Alerts (B):
- Daily digest now lists open trades with unrealized gain, trailing stop, and how
  far away it is.
- New "trade closed" alert: one summary per auto-close (trailing/target/stop, not
  manual) — direction, reason, days held, P&L abs+%/R — covering wins AND
  stop-loss losses. Deduped by trade id; toggle in Admin alerts.

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

frontend/src/pages/AdminPage.tsx

@@ -1,5 +1,6 @@
 import { useState } from 'react';
 import { ActivationSettings } from '../components/admin/ActivationSettings';
+import { ExitPolicySettings } from '../components/admin/ExitPolicySettings';
 import { AlertSettings } from '../components/admin/AlertSettings';
 import { SentimentProviderSettings } from '../components/admin/SentimentProviderSettings';
 import { DataCleanup } from '../components/admin/DataCleanup';
@@ -33,6 +34,7 @@ export default function AdminPage() {
         {activeTab === 'Settings' && (
           <div className="space-y-4">
             <ActivationSettings />
+            <ExitPolicySettings />
             <AlertSettings />
             <SentimentProviderSettings />
             <TickerUniverseBootstrap />