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/app/timeutil.py

@@ -1,4 +1,17 @@
-from datetime import datetime, timezone
+from datetime import date, datetime, timedelta, timezone
+
+# Costa Rica has no DST; a fixed offset is exact year-round.
+CR_TZ = timezone(timedelta(hours=-6))
+
+
+def today_cr() -> date:
+    """Current date in Costa Rica (UTC-6).
+
+    The server clock is UTC — at 6 pm in Costa Rica it is already "tomorrow"
+    in UTC. Anything user-facing that means "today" (agent answers, date
+    defaults) must come from here, never date.today().
+    """
+    return datetime.now(CR_TZ).date()
 
 
 def utcnow() -> datetime: