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/components/settings/automation.tsx

@@ -43,7 +43,7 @@ const CACHE_CARD: CardSpec = {
 const BUDGET_CARD: CardSpec = {
   anchor: "budget",
   title: "每日预算",
-  description: "任一项到上限后不再建新的下载计划；前两项按自然日计。",
+  description: "任一项到上限后不再开始新的下载；前两项按自然日计。",
   fields: [
     { name: "budget.maxNewTitlesPerDay", label: "每日新作品数", kind: "number" },
     { name: "budget.maxDownloadGiBPerDay", label: "每日下载量(GiB)", kind: "number" },