feat(config): add expired box cleanup functionality

Adds dedicated config setting and cleanup logic for
managing expired content boxes via admin tools and CLI.

lib/server/server.go

@@ -104,6 +104,7 @@ func Run(addr string) error {
 	compressed.Static("/static", "./static")
 
 	boxstore.StartThumbnailWorker(cfg.ThumbnailBatchSize, time.Duration(cfg.ThumbnailIntervalSeconds)*time.Second)
+	app.startExpiredCleanupWorker()
 
 	return router.Run(addr)
 }