Sync Status: per-source ingestion health page with nav warning badge

The n8n flows fail invisibly from the app's perspective (review UX-17,
the top trust gap). /api/v1/sync-status derives last-received + cadence
thresholds for BAC, salary, municipal, pensions and exchange rate from
existing data — no n8n integration needed. New Sincronización page
shows per-source freshness and a hint to check n8n; the sidebar item
gets an amber dot when any source is stale. Verified live against the
dev DB (correctly flags its stale BAC data at 43 days).

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

frontend/src/App.tsx

@@ -23,6 +23,7 @@ import Salarios from "./pages/Salarios";
 import Pensions from "./pages/Pensions";
 import Proyecciones from "./pages/Proyecciones";
 import ServiciosMunicipales from "./pages/ServiciosMunicipales";
+import SyncStatus from "./pages/SyncStatus";
 
 function ProtectedRoute({ children }: { children: React.ReactNode }) {
   const { isAuthenticated, isLoading } = useAuth();
@@ -56,6 +57,7 @@ function AppRoutes() {
         <Route path="/salarios" element={<Salarios />} />
         <Route path="/pensions" element={<Pensions />} />
         <Route path="/servicios-municipales" element={<ServiciosMunicipales />} />
+        <Route path="/sync" element={<SyncStatus />} />
         <Route path="/transactions" element={<Navigate to="/budget" replace />} />
         <Route path="/transfers" element={<Navigate to="/budget" replace />} />
       </Route>