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/lib/types.ts

@@ -207,6 +207,14 @@ export interface PaperTrade {
   benchmark_return_pct: number | null;
   alpha_pct: number | null;
   alpha_usd: number | null;
+  close_reason: 'trailing' | 'stop' | 'target' | 'manual' | null;
+  trailing_stop: number | null;
+  trailing_distance_pct: number | null;
+}
+
+export interface ExitPolicy {
+  mode: 'trailing' | 'target';
+  trailing_pct: number;
 }
 
 export interface BacktestBucket {
@@ -407,6 +415,7 @@ export interface AlertConfig {
   score_drop_enabled: boolean;
   digest_enabled: boolean;
   regime_quadrant_enabled: boolean;
+  trade_closed_enabled: boolean;
 }
 
 export interface AlertTestResult {