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/hooks/usePaperTrades.ts

@@ -1,5 +1,6 @@
 import { useMutation, useQuery, useQueryClient } from '@tanstack/react-query';
 import * as api from '../api/paperTrades';
+import type { ExitPolicy } from '../lib/types';
 import { useToast } from '../components/ui/Toast';
 
 export function usePaperTrades(status?: 'open' | 'closed') {
@@ -10,6 +11,27 @@ export function usePaperTrades(status?: 'open' | 'closed') {
   });
 }
 
+export function useExitPolicy() {
+  return useQuery({
+    queryKey: ['paper-trades', 'exit-policy'],
+    queryFn: () => api.getExitPolicy(),
+    staleTime: 5 * 60 * 1000,
+  });
+}
+
+export function useUpdateExitPolicy() {
+  const qc = useQueryClient();
+  const { addToast } = useToast();
+  return useMutation({
+    mutationFn: (body: Partial<ExitPolicy>) => api.updateExitPolicy(body),
+    onSuccess: () => {
+      qc.invalidateQueries({ queryKey: ['paper-trades'] });
+      addToast('success', 'Exit policy updated.');
+    },
+    onError: (e: Error) => addToast('error', e.message || 'Failed to update exit policy'),
+  });
+}
+
 export function useCreatePaperTrade() {
   const qc = useQueryClient();
   const { addToast } = useToast();