feat(metrics): real Sharpe ratio from daily PnL curve with minimum-sample gate

sharpe_ratio was hardcoded to 0.0 in MetricsTracker and exposed as
'or 0' in /api/summary. With only 1 resolved trade (~40 flat days plus
one +299 jump) any computed Sharpe is statistically meaningless, so:

- bot/metrics/sharpe.py: annualized Sharpe (sqrt(365)) from daily
  total_pnl closes, normalized by bankroll; sharpe_with_gate() returns
  None + status until >=30 days observed AND >=10 resolved trades.
- Database.get_daily_pnl_closes(): last metrics_daily snapshot per UTC
  day, oldest first — the return series input.
- MetricsTracker: stores the real (gated) Sharpe in the snapshot, NULL
  below the gate; log line now includes sharpe.
- /api/summary: live Sharpe + sharpe_status/days_observed/min_* fields
  explaining why it is null; resolved_count now live from COUNT(*).
- promotion_ready: requires resolved>=10, days>=30, and non-null
  win_rate/calibration/sharpe plus existing thresholds — a single lucky
  resolved trade can no longer promote.
- Dashboard Sharpe card shows the insufficient-sample explanation when
  null instead of a bare em dash.

Tests: 13 new in tests/test_sharpe_gate.py (formula, gate, API contract,
tracker snapshot); verified failing pre-fix. Suite: 62 passed.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

dashboard/src/App.jsx

@@ -200,8 +200,12 @@ export default function App() {
         <MetricCard
           title="Sharpe"
           value={fmt(summary.sharpe_ratio)}
-          subtitle="Objetivo ≥ 0.5"
-          progress={Math.min(1, summary.sharpe_ratio / 2)}
+          subtitle={
+            summary.sharpe_ratio == null
+              ? `Muestra insuficiente: ${summary.resolved_count}/${summary.min_resolved_required} resueltos, ${summary.days_observed}/${summary.min_days_required} días`
+              : 'Objetivo ≥ 0.5'
+          }
+          progress={summary.sharpe_ratio == null ? 0 : Math.min(1, summary.sharpe_ratio / 2)}
           progressColor={summary.sharpe_ratio >= 0.5 ? 'var(--green)' : 'var(--amber)'}
         />
         <MetricCard