fix(cron): add timezone support for accurate next run time calculation

When schedule.tz is present, use the specified timezone to calculate the next execution time, ensuring scheduled tasks trigger correctly across different timezones.

nanobot/cron/service.py

@@ -4,6 +4,7 @@ import asyncio
 import json
 import time
 import uuid
+from datetime import datetime
 from pathlib import Path
 from typing import Any, Callable, Coroutine
 
@@ -30,9 +31,18 @@ def _compute_next_run(schedule: CronSchedule, now_ms: int) -> int | None:
     if schedule.kind == "cron" and schedule.expr:
         try:
             from croniter import croniter
-            cron = croniter(schedule.expr, time.time())
-            next_time = cron.get_next()
-            return int(next_time * 1000)
+            from zoneinfo import ZoneInfo
+            base_time = time.time()
+            if schedule.tz:
+                tz = ZoneInfo(schedule.tz)
+                base_dt = datetime.fromtimestamp(base_time, tz=tz)
+                cron = croniter(schedule.expr, base_dt)
+                next_dt = cron.get_next(datetime)
+                return int(next_dt.timestamp() * 1000)
+            else:
+                cron = croniter(schedule.expr, base_time)
+                next_time = cron.get_next()
+                return int(next_time * 1000)
         except Exception:
             return None