feat(roles): add rclone-mount role for WebDAV mounts

Add a new system-level role for mounting WebDAV filesystems via rclone.
Includes rclone-torbox-setup helper script that uses rbw to bootstrap
credentials from Bitwarden.

Key features:
- Configurable WebDAV URL, username, mount point
- VFS cache mode and buffer size tuning for media streaming
- RequiresMountsFor option for ZFS pool dependencies
- Obscured password storage via environment file

Enable on john-endesktop for TorBox WebDAV access by rdt-client and
Jellyfin. Mount waits for /media ZFS pool before starting.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

packages/default.nix

@@ -4,4 +4,5 @@
   app-launcher-server = pkgs.callPackage ./app-launcher-server {};
   claude-code = pkgs.callPackage ./claude-code {};
   mcrcon-rbw = pkgs.callPackage ./mcrcon-rbw {};
+  rclone-torbox-setup = pkgs.callPackage ./rclone-torbox-setup {};
 }