feat(downloads): fold download plans into download panel (DL-3)

- Waiting plans (approval / disk space / automation off / queued) are
  active-segment rows on /download-tasks/board, merged by created_at,
  counted, with approve / cancel actions; ?plan= deep links and legacy
  /downloads/plans routes redirect to the panel
- Download detail shows "why this download" (source, purpose, replaced
  version, reasons); raw evaluation in tech info
- Failed / cancelled / expired plans appear in the entity download
  history; rejected / revoked / expired approvals close draft plans
- New GET /acquisition-plans/{id}/why, POST /acquisition-plans/{id}/cancel
- User-facing copy no longer says 下载计划; glossary updated

Co-Authored-By: Claude Opus 5.5 <[email protected]>

apps/web/app/api/tasks.ts

@@ -173,7 +173,7 @@ export function buildTimeline(input: {
       key: "plan",
       stage: "plan",
       at: plan.createdAt,
-      title: `创建下载计划(${plan.purpose === "fill" ? "补缺" : plan.purpose === "upgrade" ? "升级" : "换源"})`,
+      title: `准备下载(${plan.purpose === "fill" ? "补缺" : plan.purpose === "upgrade" ? "升级" : "换源"})`,
       detail: humanizeReason(plan.statusReason),
       tone: "neutral",
     });
@@ -182,7 +182,7 @@ export function buildTimeline(input: {
         key: "plan-status",
         stage: "plan",
         at: plan.updatedAt,
-        title: `计划状态：${PLAN_STATUS_LABEL[plan.status]}`,
+        title: `下载准备：${PLAN_STATUS_LABEL[plan.status]}`,
         detail: humanizeReason(plan.statusReason),
         tone: plan.status === "failed" ? "destructive" : plan.status === "expired" ? "warning" : "neutral",
       });