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/tests/test_sync_and_export.py

@@ -0,0 +1,112 @@
+"""Sync-status derivation (UX-17) and CSV export (ARCH-18)."""
+
+import csv
+import io
+from datetime import datetime, timedelta
+from decimal import Decimal
+
+from app.models.models import (
+    Bank,
+    Category,
+    Currency,
+    ExchangeRate,
+    Transaction,
+    TransactionSource,
+    TransactionType,
+)
+from app.services.csv_export import build_transactions_csv
+from app.services.sync_status import compute_sync_status
+from app.timeutil import utcnow
+
+
+def by_key(statuses: list[dict]) -> dict[str, dict]:
+    return {s["key"]: s for s in statuses}
+
+
+class TestSyncStatus:
+    def test_empty_db_reports_never(self, session):
+        statuses = by_key(compute_sync_status(session))
+        assert len(statuses) == 5
+        assert all(s["status"] == "never" for s in statuses.values())
+        assert all(s["last_received"] is None for s in statuses.values())
+
+    def test_fresh_data_is_ok_and_stale_warns(self, session):
+        fresh = Transaction(
+            amount=Decimal("1"),
+            merchant="X",
+            date=utcnow(),
+            source=TransactionSource.CREDIT_CARD,
+            bank=Bank.BAC,
+        )
+        fresh.created_at = utcnow() - timedelta(days=1)
+        stale_salary = Transaction(
+            amount=Decimal("1"),
+            merchant="EMPLOYER",
+            date=utcnow(),
+            transaction_type=TransactionType.SALARY,
+            source=TransactionSource.TRANSFER,
+        )
+        stale_salary.created_at = utcnow() - timedelta(days=60)
+        session.add(fresh)
+        session.add(stale_salary)
+        session.add(
+            ExchangeRate(
+                date=utcnow(),
+                buy_rate=Decimal("500"),
+                sell_rate=Decimal("510"),
+            )
+        )
+        session.commit()
+
+        statuses = by_key(compute_sync_status(session))
+        assert statuses["bac_credit_card"]["status"] == "ok"
+        assert statuses["bac_credit_card"]["age_days"] == 1.0
+        assert statuses["salary"]["status"] == "warning"
+        assert statuses["exchange_rate"]["status"] == "ok"
+        assert statuses["municipal"]["status"] == "never"
+
+
+class TestCsvExport:
+    def _seed(self, session):
+        cat = Category(name="Food")
+        session.add(cat)
+        session.commit()
+        session.refresh(cat)
+        session.add(
+            Transaction(
+                amount=Decimal("1500.50"),
+                merchant="SODA, LA \"BUENA\"",  # exercises CSV quoting
+                date=datetime(2026, 4, 2),
+                category_id=cat.id,
+                currency=Currency.CRC,
+            )
+        )
+        session.add(
+            Transaction(
+                amount=Decimal("10"),
+                merchant="CASHTHING",
+                date=datetime(2026, 4, 3),
+                source=TransactionSource.CASH,
+            )
+        )
+        session.commit()
+
+    def test_full_export_with_quoting_and_category(self, session):
+        self._seed(session)
+        text = build_transactions_csv(session)
+        rows = list(csv.reader(io.StringIO(text)))
+        assert rows[0][:3] == ["id", "date", "merchant"]
+        assert len(rows) == 3  # header + 2
+        newest_first = rows[1]
+        assert newest_first[2] == "CASHTHING"
+        soda = rows[2]
+        assert soda[2] == 'SODA, LA "BUENA"'  # round-trips through quoting
+        assert soda[4] == "1500.50"
+        assert soda[9] == "Food"
+
+    def test_source_filter(self, session):
+        self._seed(session)
+        text = build_transactions_csv(session, source=TransactionSource.CASH)
+        rows = list(csv.reader(io.StringIO(text)))
+        assert len(rows) == 2
+        assert rows[1][2] == "CASHTHING"