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

reaction_board/timeutil.py

@@ -57,3 +57,39 @@ def day_bounds(date: str) -> tuple[float, float]:
         raise ValueError(f"日期格式必须是 YYYY-MM-DD: {date!r}")
     start = datetime.strptime(date, "%Y-%m-%d").replace(tzinfo=SHANGHAI_TZ)
     return start.timestamp(), (start + timedelta(days=1)).timestamp()
+
+
+# 全站快照静默窗：上海时区 [23:00, 01:00)。此间 discovery / recent / cleanup 不执行。
+SNAPSHOT_QUIET_START_HOUR = 23
+SNAPSHOT_QUIET_END_HOUR = 1
+
+
+def in_snapshot_quiet_window(ts: float | None = None) -> bool:
+    """是否处于全站快照静默窗 [23:00, 01:00)（上海时区，跨日）。"""
+    dt = shanghai_datetime(ts)
+    return dt.hour >= SNAPSHOT_QUIET_START_HOUR or dt.hour < SNAPSHOT_QUIET_END_HOUR
+
+
+def next_after_snapshot_quiet_window(ts: float) -> float:
+    """若 ``ts`` 落在静默窗内，推到窗结束后的 01:00；否则原样返回。"""
+    if not in_snapshot_quiet_window(ts):
+        return ts
+    dt = shanghai_datetime(ts)
+    if dt.hour >= SNAPSHOT_QUIET_START_HOUR:
+        target = (dt + timedelta(days=1)).replace(
+            hour=SNAPSHOT_QUIET_END_HOUR, minute=0, second=0, microsecond=0,
+        )
+    else:
+        target = dt.replace(
+            hour=SNAPSHOT_QUIET_END_HOUR, minute=0, second=0, microsecond=0,
+        )
+    if target.timestamp() <= ts:
+        target += timedelta(days=1)
+    return target.timestamp()
+
+
+def interval_next_run(now: float, interval: float) -> float:
+    """``now + interval``，若落入静默窗则推到 01:00 之后。"""
+    if interval <= 0:
+        raise ValueError("interval 必须大于 0")
+    return next_after_snapshot_quiet_window(now + interval)