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/endpoints/sync_status.py

@@ -0,0 +1,20 @@
+from fastapi import APIRouter, Depends
+from sqlmodel import Session
+
+from app.auth import get_current_user
+from app.db import get_session
+from app.services.sync_status import compute_sync_status
+
+router = APIRouter(prefix="/sync-status", tags=["sync-status"])
+
+
+@router.get("/")
+def sync_status(
+    session: Session = Depends(get_session),
+    _user: str = Depends(get_current_user),
+):
+    sources = compute_sync_status(session)
+    return {
+        "sources": sources,
+        "warnings": sum(1 for s in sources if s["status"] in ("warning", "never")),
+    }