Add activation thresholds: qualified-signal defaults and views

Admin-configurable thresholds (min R:R, default 2.0; min confidence,
default 70%) defining what counts as an actionable signal:

- Admin Settings: new Activation Thresholds panel
  (GET/PUT /admin/settings/activation)
- GET /trades/activation exposes values to all users with access
- Signals/Setups: filters initialize from activation values
- Track Record: "Qualified signals only" toggle (default on) via
  min_rr/min_confidence params on /trades/performance; the
  confidence breakdown always covers the full population so the
  thresholds can be validated against outcomes
- Dashboard: "Qualified" metric and qualified-first Top Setups
- Outcome evaluator unchanged: every setup is still evaluated

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

frontend/src/hooks/useActivation.ts

@@ -0,0 +1,10 @@
+import { useQuery } from '@tanstack/react-query';
+import { getActivation } from '../api/activation';
+
+export function useActivation() {
+  return useQuery({
+    queryKey: ['activation'],
+    queryFn: getActivation,
+    staleTime: 5 * 60 * 1000,
+  });
+}