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/api/paperTrades.ts

@@ -1,5 +1,5 @@
 import apiClient from './client';
-import type { PaperTrade } from '../lib/types';
+import type { ExitPolicy, PaperTrade } from '../lib/types';
 
 export function listPaperTrades(status?: 'open' | 'closed') {
   return apiClient
@@ -7,6 +7,14 @@ export function listPaperTrades(status?: 'open' | 'closed') {
     .then((r) => r.data);
 }
 
+export function getExitPolicy() {
+  return apiClient.get<ExitPolicy>('paper-trades/exit-policy').then((r) => r.data);
+}
+
+export function updateExitPolicy(payload: Partial<ExitPolicy>) {
+  return apiClient.put<ExitPolicy>('paper-trades/exit-policy', payload).then((r) => r.data);
+}
+
 export interface CreatePaperTradeBody {
   symbol: string;
   direction: 'long' | 'short';