feat: implement rule management (US3) + dashboard (US4)

US3 - Rule Management:
- Backend: CRUD API + activate/deactivate + test against 90d history
- Frontend: RuleList sidebar, RuleForm (create/edit), RuleTestResults
- Weight/threshold sliders, condition type selector, audit logging

US4 - Dashboard:
- Backend: GET /dashboard/metrics (24h/7d/30d/90d), CSV export
- Frontend: MetricCards, alerts-by-hour bar chart, status pie chart,
  top rules horizontal bar chart via Recharts

Stack: 20 API routes total. Frontend tsc clean, vite build passes.

frontend/src/api/client.ts

@@ -85,11 +85,29 @@ export interface TransactionAccepted {
 }
 
 export interface DashboardMetrics {
+  period: string;
   total_transactions: number;
   total_alerts: number;
   fraud_rate_pct: number;
   false_positive_rate_pct: number;
   alerts_by_status: { pending: number; confirmed: number; false_positive: number; escalated: number };
+  alerts_by_hour: { hour: number; count: number }[];
+  top_triggering_rules: { rule_name: string; alert_count: number }[];
+  score_distribution: { low: number; medium: number; high: number; critical: number };
+}
+
+export interface DetectionRule {
+  id: string;
+  name: string;
+  description: string;
+  condition: string;
+  weight: number;
+  threshold: number;
+  is_active: boolean;
+  version: number;
+  created_by: string;
+  created_at: string;
+  updated_at: string;
 }
 
 export function listAlerts(params: AlertFilters = {}): Promise<AlertListResponse> {