feat: backup, notifier, organizer, douban, watch-history, poster-wall

Complete the port of the remaining MediaStation features:

Backend services:
  - service/backup.go: SQLite hot backup via VACUUM INTO, list, delete,
    restore. Backup files stored under {data_dir}/backups/ with timestamps.
  - service/notifier.go: Multi-channel push notifications (Telegram /
    Bark / WeChat Server酱 / Webhook). Configuration via settings table
    keys notify.telegram.*, notify.bark.*, etc.
  - service/organizer.go: Auto-rename + move media files into library
    directory structure. Movie: {Title} ({Year})/{Title} ({Year}).ext,
    TV: {Title}/Season XX/{Title} - SxxExx.ext. Both per-media and
    per-library batch endpoints.
  - service/douban.go: Douban (豆瓣) metadata provider using the
    unofficial subject_suggest API. Returns Chinese titles + posters.
    Requires douban_cookie in secrets config.

Handlers:
  - handler/backup.go: GET/POST/DELETE /admin/backups, POST restore.
  - handler/organizer.go: POST /admin/media/:id/organize,
    POST /admin/libraries/:id/organize.
  - handler/notify.go: POST /admin/notify/test.

Frontend pages:
  - WatchHistoryPage.tsx: full paginated history with progress bars,
    resume buttons, poster thumbnails and timestamps.
  - PosterWallPage.tsx: dense 8-column poster grid of all media
    (matches the original MediaStation PosterWallView.vue).

Sidebar additions: 观看历史 + 海报墙 links in the main nav group.

Verified: go build, go vet, go test all pass; tsc -b && vite build
emits 30 lazy chunks; main bundle 254 KB / 85 KB gzipped.

web/src/components/Layout.tsx

@@ -12,6 +12,7 @@ import {
   HardDrive,
   Heart,
   Home,
+  GalleryHorizontalEnd,
   KeyRound,
   ListChecks,
   ListMusic,
@@ -61,6 +62,8 @@ export function Layout() {
           <SidebarLink to="/search" icon={<Search size={18} />} label="搜索" />
           <SidebarLink to="/favourites" icon={<Heart size={18} />} label="收藏" />
           <SidebarLink to="/playlists" icon={<ListMusic size={18} />} label="播放列表" />
+          <SidebarLink to="/history" icon={<Clock size={18} />} label="观看历史" />
+          <SidebarLink to="/poster-wall" icon={<GalleryHorizontalEnd size={18} />} label="海报墙" />
 
           <div className="mt-6 px-2 text-xs uppercase tracking-wider text-slate-500">
             媒体库