fix(scheduler): drop cross-task mutex, quiet window + retry

Incident: evening full snapshot could be recorded as ok with
already_completed/elapsed=0 after mutual skip with hourly sync, then
next_run jumped to the next day.

- Full-site snapshot never skips because another task is running
- 23:00-01:00 Asia/Shanghai: defer discovery / snapshot_recent /
  candidate_cleanup to after 01:00; fans never defers
- Skip records ok=false + skipped=true (not success / not completed)
- Skip/fail reschedule soon (~5m); daily success still next snapshot_at
- TaskControl return value drives next_run; system UI shows skip fields

tests/test_timeutil.py

@@ -11,7 +11,10 @@ sys.stdout.reconfigure(encoding="utf-8")
 from reaction_board.timeutil import (  # noqa: E402
     SHANGHAI_TZ,
     day_bounds,
+    in_snapshot_quiet_window,
+    interval_next_run,
     is_date,
+    next_after_snapshot_quiet_window,
     next_daily_run,
     parse_hhmm,
     shanghai_date,
@@ -72,11 +75,40 @@ def test_day_bounds_and_date_validation():
         raise AssertionError("非法日期应失败")
 
 
+
+def test_snapshot_quiet_window():
+    assert in_snapshot_quiet_window(ts("2026-11-02 23:00:00"))
+    assert in_snapshot_quiet_window(ts("2026-11-02 23:59:59"))
+    assert in_snapshot_quiet_window(ts("2026-11-03 00:00:00"))
+    assert in_snapshot_quiet_window(ts("2026-11-03 00:59:59"))
+    assert not in_snapshot_quiet_window(ts("2026-11-03 01:00:00"))
+    assert not in_snapshot_quiet_window(ts("2026-11-02 22:59:59"))
+    assert not in_snapshot_quiet_window(ts("2026-11-02 12:00:00"))
+
+    assert next_after_snapshot_quiet_window(ts("2026-11-02 22:00:00")) == ts(
+        "2026-11-02 22:00:00"
+    )
+    assert next_after_snapshot_quiet_window(ts("2026-11-02 23:15:00")) == ts(
+        "2026-11-03 01:00:00"
+    )
+    assert next_after_snapshot_quiet_window(ts("2026-11-03 00:30:00")) == ts(
+        "2026-11-03 01:00:00"
+    )
+    # 22:50 + 60min = 23:50 → 推到次日 01:00
+    assert interval_next_run(ts("2026-11-02 22:50:00"), 3600) == ts(
+        "2026-11-03 01:00:00"
+    )
+    assert interval_next_run(ts("2026-11-02 12:00:00"), 3600) == ts(
+        "2026-11-02 13:00:00"
+    )
+
+
 def main():
     test_shanghai_date_boundary()
     test_parse_hhmm()
     test_next_daily_run()
     test_day_bounds_and_date_validation()
+    test_snapshot_quiet_window()
     print("OK: 上海时区与每日时间计算测试全部通过")