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.

internal/handler/handler.go

@@ -142,6 +142,19 @@ func Register(r *gin.Engine, cfg *config.Config, log *zap.Logger, svc *service.C
 			admin.PUT("/settings", updateSettingHandler(svc))
 			admin.GET("/logs", recentLogsHandler(svc))
 
+			// Database backup.
+			admin.GET("/backups", listBackupsHandler(svc))
+			admin.POST("/backups", createBackupHandler(svc))
+			admin.DELETE("/backups", deleteBackupHandler(svc))
+			admin.POST("/backups/restore", restoreBackupHandler(svc))
+
+			// Notifications (test endpoint).
+			admin.POST("/notify/test", notifyTestHandler(svc))
+
+			// File organizer.
+			admin.POST("/media/:id/organize", organizeMediaHandler(svc))
+			admin.POST("/libraries/:id/organize", organizeLibraryHandler(svc))
+
 			// API key management (encrypted at rest).
 			admin.GET("/api-configs", listAPIConfigsHandler(svc))
 			admin.GET("/api-configs/:provider", getAPIConfigHandler(svc))