feat(admin): purge-archived endpoint with an archived-items banner

Bulk escape hatch for archived rows (files missing from disk for 2+
scans) so a mass external deletion never has to wait out the retention
window or be clicked away row by row:

- POST /api/media-items/purge-archived (admin only) hard-deletes all
  archived items and returns the purged count; reading history goes with
  the rows, so the call is confirmed in the UI first.
- The library admin page shows an 'Archived items: N' card (only when
  non-zero) with a Purge Archived Now button that calls the endpoint,
  toasts the result, and reloads.
- Frontend admin JS exposes window.purgeArchivedItems following the
  existing localStorage-bearer-token pattern.

internal/router/media.go

@@ -62,6 +62,7 @@ func registerMediaRoutes(cfg *Config) {
 	admin.PUT("/media-items/:id", cfg.MediaHandler.UpdateMediaItem)
 	admin.POST("/media-items/:id/rescan", cfg.MediaHandler.RescanMediaItem)
 	admin.DELETE("/media-items/:id", cfg.MediaHandler.DeleteMediaItem)
+	admin.POST("/media-items/purge-archived", cfg.MediaHandler.PurgeArchivedMediaItems)
 
 	// Shelf management (protected)
 	protected.POST("/devices/:id/shelves", cfg.MediaHandler.AddToShelf)