feat: M14 — Observability (dashboard charts, agent fleet, stats API, metrics, structured logging, rollback)

Backend: StatsService with 5 aggregation methods, JSON metrics endpoint, slog-based
structured logging middleware. Stats API: dashboard summary, certificates-by-status,
expiration timeline, job trends, issuance rate. 23 new backend tests.

Frontend: Recharts-powered dashboard with 4 charts (status pie, expiration heatmap,
job trends line, issuance bar), agent fleet overview page with OS/arch grouping and
version breakdown, deployment rollback buttons on version history. 7 new frontend tests.

78 API endpoints, 744+ total tests (658 Go + 86 Vitest).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/integration/negative_test.go

@@ -69,6 +69,8 @@ func setupTestServer(t *testing.T) (*httptest.Server, *mockCertificateRepository
 	agentGroupHandler := handler.NewAgentGroupHandler(&mockAgentGroupService{})
 	auditHandler := handler.NewAuditHandler(auditService)
 	notificationHandler := handler.NewNotificationHandler(notificationService)
+	statsHandler := handler.NewStatsHandler(&mockStatsService{})
+	metricsHandler := handler.NewMetricsHandler(&mockStatsService{}, time.Now())
 	healthHandler := handler.NewHealthHandler("none")
 
 	r := router.New()
@@ -85,6 +87,8 @@ func setupTestServer(t *testing.T) (*httptest.Server, *mockCertificateRepository
 		agentGroupHandler,
 		auditHandler,
 		notificationHandler,
+		statsHandler,
+		metricsHandler,
 		healthHandler,
 	)