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/web/static/views/system.js

@@ -5,6 +5,15 @@ import {
   escapeHtml, formatCount, formatDateTime, formatNumber,
 } from "../format.js";
 
+function taskResultLabel(record) {
+  const summary = record.summary && typeof record.summary === "object" ? record.summary : {};
+  if (record.skipped || summary.skipped) {
+    return `<span class="muted">跳过</span>`;
+  }
+  if (record.ok) return "成功";
+  return `<span class="danger">失败</span>`;
+}
+
 function taskRows(tasks) {
   const entries = Object.entries(tasks || {});
   if (!entries.length) return emptyState("暂无任务记录", "运行一次同步、快照或发现后会写入最近结果。");
@@ -15,12 +24,16 @@ function taskRows(tasks) {
         ${entries.map(([name, record]) => {
           const summary = record.summary && typeof record.summary === "object"
             ? Object.entries(record.summary)
-              .filter(([key]) => ["added", "removed", "candidates", "notified", "ok", "failed", "total", "completed", "backup_error"].includes(key))
+              .filter(([key]) => [
+                "added", "removed", "candidates", "notified", "ok", "failed",
+                "total", "completed", "backup_error", "skipped", "already_completed",
+                "reason", "elapsed", "success",
+              ].includes(key))
               .map(([key, value]) => `${key}=${value}`).join(" ")
             : "";
           return `<tr>
             <td>${escapeHtml(name)}</td>
-            <td>${record.ok ? "成功" : `<span class="danger">失败</span>`}</td>
+            <td>${taskResultLabel(record)}</td>
             <td>${escapeHtml(formatDateTime(record.finished_at))}</td>
             <td class="summary-cell">${escapeHtml(summary || record.error || "—")}</td>
           </tr>`;
@@ -100,7 +113,7 @@ export async function renderSystem(root) {
         <div><span>新视频小时同步</span><strong>${running.snapshot_recent ? "运行中" : escapeHtml(formatDateTime(next.snapshot_recent))}</strong></div>
         <div><span>新视频搜索</span><strong>${running.discover || running.discovery ? "运行中" : escapeHtml(formatDateTime(next.discover))}</strong></div>
       </div>
-      <p class="section-note">进行中显示「运行中」；空闲时按该任务上次完成时间推算下次运行，不按墙钟抢跑。新视频小时同步与全站快照互斥。</p>
+      <p class="section-note">进行中显示「运行中」；空闲时按该任务上次完成时间推算下次运行。23:00–01:00（上海）为全站快照窗：发现 / 小时同步 / 候选清理会延后到 01:00 后；粉丝更新不避开。跳过或失败不会记成成功，会短间隔补跑。</p>
     </section>
 
     <section class="panel">