Agent: fix date handling — Costa Rica timezone and future cuotas

Two bugs from prod (server clock is UTC, user is UTC-6):

- get_recent_transactions returned future-dated Tasa Cero cuotas; it now
  excludes anything dated after the user's today, same rule as
  /transactions/recent.
- "¿Cuánto he gastado hoy?" answered for the wrong day: the prompt baked
  date.today() in at boot — frozen from startup AND in server-UTC (8 pm
  in Costa Rica is already tomorrow in UTC). The prompt no longer carries
  a date; a new get_current_date tool returns today in CR time plus the
  active billing cycle, and the prompt directs the model to call it for
  any relative date reference. today_cr() lives in timeutil (CR has no
  DST, fixed UTC-6).

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

backend/tests/test_agent_tools.py

@@ -149,3 +149,37 @@ def test_recent_transactions_json_safe(bound_session):
     out = tools.get_recent_transactions()
     json.dumps(out)
     assert out[0]["amount"] == 42.42
+
+
+def test_recent_transactions_exclude_future_cuotas(bound_session):
+    from datetime import timedelta
+
+    from app.timeutil import today_cr
+
+    today = datetime.combine(today_cr(), datetime.min.time())
+    bound_session.add(
+        Transaction(amount=Decimal("10"), merchant="BILLED", date=today)
+    )
+    bound_session.add(
+        Transaction(
+            amount=Decimal("20"),
+            merchant="FUTURE CUOTA:03/06",
+            date=today + timedelta(days=45),
+        )
+    )
+    bound_session.commit()
+    merchants = [t["merchant"] for t in tools.get_recent_transactions()]
+    assert "BILLED" in merchants
+    assert "FUTURE CUOTA:03/06" not in merchants
+
+
+def test_get_current_date_is_costa_rica_and_cycle_consistent():
+    from app.timeutil import today_cr
+
+    out = tools.get_current_date()
+    today = today_cr()
+    assert out["date"] == today.isoformat()
+    # The active cycle must contain today: [start, end)
+    start, end = out["cycle_range"]
+    assert start <= out["date"] < end
+    assert start.endswith("-18") and end.endswith("-18")