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/lib/api.ts

@@ -485,3 +485,22 @@ export const getWaterConsumption = (months?: number) =>
   api.get<WaterMeterReading[]>("/municipal-receipts/water-consumption", {
     params: months ? { months } : undefined,
   });
+
+// --- Sync Status ---
+
+export interface SyncSource {
+  key: string;
+  label: string;
+  description: string;
+  last_received: string | null;
+  age_days: number | null;
+  warn_after_days: number;
+  status: 'ok' | 'warning' | 'never';
+}
+
+export interface SyncStatusResponse {
+  sources: SyncSource[];
+  warnings: number;
+}
+
+export const getSyncStatus = () => api.get<SyncStatusResponse>('/sync-status/');