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/admin.ts

@@ -134,6 +134,8 @@ export function updateAlertSettings(payload: {
   sr_proximity_enabled?: boolean;
   score_drop_enabled?: boolean;
   digest_enabled?: boolean;
+  regime_quadrant_enabled?: boolean;
+  trade_closed_enabled?: boolean;
 }) {
   return apiClient
     .put<AlertConfig>('admin/settings/alerts', payload)