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>

backend/app/api/v1/router.py

@@ -14,6 +14,7 @@ from app.api.v1.endpoints import (
     salarios,
     savings_accrual,
     settings,
+    sync_status,
     tokens,
     transactions,
 )
@@ -34,3 +35,4 @@ api_router.include_router(salarios.router)
 api_router.include_router(pensions.router)
 api_router.include_router(municipal_receipts.router)
 api_router.include_router(savings_accrual.router)
+api_router.include_router(sync_status.router)