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>

web/src/main.tsx

@@ -22,6 +22,7 @@ import TeamsPage from './pages/TeamsPage';
 import AgentGroupsPage from './pages/AgentGroupsPage';
 import AuditPage from './pages/AuditPage';
 import ShortLivedPage from './pages/ShortLivedPage';
+import AgentFleetPage from './pages/AgentFleetPage';
 import './index.css';
 
 const queryClient = new QueryClient({
@@ -48,6 +49,7 @@ createRoot(document.getElementById('root')!).render(
                   <Route path="certificates/:id" element={<CertificateDetailPage />} />
                   <Route path="agents" element={<AgentsPage />} />
                   <Route path="agents/:id" element={<AgentDetailPage />} />
+                  <Route path="fleet" element={<AgentFleetPage />} />
                   <Route path="jobs" element={<JobsPage />} />
                   <Route path="notifications" element={<NotificationsPage />} />
                   <Route path="policies" element={<PoliciesPage />} />