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/scheduler.py

@@ -3,6 +3,9 @@
 
 P0 只注册收藏夹同步任务；P1/P3 会在同一调度器上追加快照、粉丝和发现任务。
 调度只负责“什么时候运行”，具体任务保持幂等，并在自己的服务层处理重试。
+
+任务 ``fn`` 可返回 :class:`TaskControl` 以覆盖 ``ok`` / ``next_run``
+（用于静默窗延后、失败补跑等）；未返回时仍按 ``next_run_fn`` / ``interval`` 排期。
 """
 
 from __future__ import annotations
@@ -12,6 +15,15 @@ from dataclasses import dataclass
 from typing import Any, Callable, Sequence
 
 
+@dataclass
+class TaskControl:
+    """任务可选返回值：覆盖成功标记与下次运行时间。"""
+
+    ok: bool = True
+    next_run: float | None = None
+    error: str | None = None
+
+
 @dataclass
 class ScheduledTask:
     name: str
@@ -70,23 +82,33 @@ class Scheduler:
                 continue
             started = current
             error: str | None = None
+            control: TaskControl | None = None
             try:
-                task.fn()
+                raw = task.fn()
             except Exception as exc:  # noqa: BLE001  单任务失败不终止调度
                 error = str(exc)
                 self._log("error", "scheduler.task_error", task=task.name, error=error)
             else:
+                if isinstance(raw, TaskControl):
+                    control = raw
                 self._log("info", "scheduler.task_ok", task=task.name)
             finished = self._clock()
-            if task.next_run_fn is not None:
+            if control is not None and control.next_run is not None:
+                task.next_run = float(control.next_run)
+            elif task.next_run_fn is not None:
                 task.next_run = task.next_run_fn(started)
             else:
                 task.next_run = started + task.interval
+            ok = error is None
+            if control is not None:
+                ok = bool(control.ok) and error is None
+                if control.error and error is None:
+                    error = control.error
             result = TaskResult(
                 name=task.name,
                 started_at=started,
                 finished_at=finished,
-                ok=error is None,
+                ok=ok,
                 error=error,
             )
             results.append(result)