fix: compute_next_run naive datetime treated as local time instead of UTC

When datetime.utcnow() (naive) was passed as after=, astimezone() treated
it as system-local time (Beijing), causing next_run_at to be calculated
incorrectly. Now naive datetimes are explicitly localized to UTC first.

Also replaced deprecated datetime.utcnow() with aware UTC equivalent.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/app/services/agent_schedule_service.py

@@ -25,6 +25,11 @@ def compute_next_run(cron_expression: str, after: Optional[datetime] = None, tz:
         下次执行的 UTC datetime（naive，存储时与 datetime.utcnow() 对齐）
     """
     base = after or datetime.now(timezone.utc)
+    # 确保 base 是 timezone-aware UTC（datetime.utcnow() 返回 naive，需要先 localize）
+    if base.tzinfo is None:
+        base = pytz.UTC.localize(base)
+    else:
+        base = base.astimezone(pytz.UTC)
     # 将 UTC 时间转换为目标时区，cron 表达式在目标时区下解释
     try:
         target_tz = pytz.timezone(tz)
@@ -117,7 +122,7 @@ def check_and_run_due_schedules() -> int:
     db: Optional[Session] = None
     try:
         db = SessionLocal()
-        now = datetime.utcnow()
+        now = datetime.now(timezone.utc).replace(tzinfo=None)  # naive UTC，与 DB 存储一致
 
         due_schedules = (
             db.query(AgentSchedule)