feat(media): images and recordings in backups, exports and imports (M6)

- Backup ZIP (download and automatic per-user server backups) carries the
  files of the user's own notes as media/<id><ext>; restoring creates
  them with new IDs and rewrites the blocks.
- Markdown archive puts them under attachments/ and embeds them relative
  to each note (![caption](../attachments/…)); re-importing turns them
  back into image/audio blocks.
- Obsidian import takes images and audio a note embeds on its own
  (![[file.png]], ![](path/file.png)); unused or invalid files appear as
  skipped in the preview instead of vanishing.
- Format detection and file paths moved into storage for this.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/api/handlers_account.go

@@ -156,7 +156,7 @@ func (s *Server) handleExport(w http.ResponseWriter, r *http.Request) {
 	if r.URL.Query().Get("format") == "zip" {
 		w.Header().Set("Content-Type", "application/zip")
 		w.Header().Set("Content-Disposition", fmt.Sprintf(`attachment; filename="mindsift-backup-%s.zip"`, time.Now().Format("2006-01-02")))
-		if err := storage.WriteBackupZip(exportData, w); err != nil {
+		if err := s.store.WriteBackupZip(userID, exportData, w); err != nil {
 			slog.Error("Backup-Export fehlgeschlagen", "user_id", userID, "error", err)
 		}
 		return