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/storage/backup.go

@@ -123,7 +123,7 @@ func (s *Store) WriteUserBackup(userID, dir string, now time.Time) (string, erro
 		return "", err
 	}
 	name := "mindsift-backup-" + now.Format(backupStampFmt) + ".zip"
-	return name, writeFileAtomically(filepath.Join(dir, name), func(w io.Writer) error { return WriteBackupZip(exp, w) })
+	return name, writeFileAtomically(filepath.Join(dir, name), func(w io.Writer) error { return s.WriteBackupZip(userID, exp, w) })
 }
 
 func gzipFile(src, dst string) error {