fix(scheduler): normalize once-schedule next_run_at to UTC (#4607)

The once branch of next_run_at returned the run time with the task's
local timezone offset attached, while the cron branch normalizes to UTC.
The value is persisted into scheduled_tasks.next_run_at, and SQLAlchemy's
SQLite dialect discards tzinfo on bind, so a once task declared in a
non-UTC timezone fires shifted by the whole offset (e.g. 8 hours late
for Asia/Shanghai, hours early for negative offsets - also bypassing the
min_once_delay_seconds guard). Postgres timestamptz normalizes on write,
which is why only SQLite deployments are affected.

Align the once branch with the cron branch by converting to UTC before
returning.

backend/packages/harness/deerflow/scheduler/schedules.py

@@ -41,6 +41,10 @@ def next_run_at(
             # A naive run_at means "wall-clock time in the task's declared
             # timezone", matching how cron schedules interpret it.
             run_at = run_at.replace(tzinfo=ZoneInfo(timezone_name))
+        # Normalize to UTC like the cron branch: next_run_at is persisted to
+        # timezone-discarding columns (SQLite), where a non-UTC offset shifts
+        # the effective fire time by the whole offset.
+        run_at = run_at.astimezone(UTC)
         return run_at if run_at > now else None
 
     if schedule_type == "cron":

backend/tests/test_scheduled_task_schedules.py

@@ -1,4 +1,4 @@
-from datetime import UTC, datetime
+from datetime import UTC, datetime, timedelta
 
 import pytest
 
@@ -38,6 +38,30 @@ def test_next_run_at_for_once_returns_none_after_fire_time():
     assert result is None
 
 
+def test_next_run_at_for_once_normalizes_naive_run_at_to_utc():
+    now = datetime(2026, 7, 31, 0, 0, tzinfo=UTC)
+    result = next_run_at(
+        "once",
+        {"run_at": "2026-08-01T09:00:00"},
+        "Asia/Shanghai",
+        now=now,
+    )
+    assert result == datetime(2026, 8, 1, 1, 0, tzinfo=UTC)
+    assert result.utcoffset() == timedelta(0)
+
+
+def test_next_run_at_for_once_normalizes_aware_run_at_to_utc():
+    now = datetime(2026, 7, 31, 0, 0, tzinfo=UTC)
+    result = next_run_at(
+        "once",
+        {"run_at": "2026-08-01T09:00:00+08:00"},
+        "UTC",
+        now=now,
+    )
+    assert result == datetime(2026, 8, 1, 1, 0, tzinfo=UTC)
+    assert result.utcoffset() == timedelta(0)
+
+
 def test_next_run_at_for_cron_uses_timezone():
     now = datetime(2026, 7, 1, 0, 30, tzinfo=UTC)
     result = next_run_at(