The derived `*_base_path` defaults concatenated `matrix_bot_maubot_path_prefix`
directly, producing `//v1` and `//plugin/` when users set the documented
`matrix_bot_maubot_path_prefix: /` (for serving on a dedicated subdomain),
which Traefik rejects. Apply the standard `'/' == path_prefix` guard already
used by other roles (honoroit, mautrix-discord, MAS, heisenbridge, etc.).

Reported by The Dark Wizard.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>