Continuwuity has no native enable-captcha toggle; it enables the ReCAPTCHA
registration flow based on the presence of a private site key. The playbook
previously always rendered empty `recaptcha_site_key`/`recaptcha_private_site_key`
values, which made Continuwuity enable a broken captcha flow and break
registration in some clients.

The keys are now only rendered when both are configured, gated by a derived
`matrix_continuwuity_recaptcha_enabled` flag in the role's `vars/main.yml`. A
consistency check fails the play when exactly one of the two keys is set.

Fixes #5329

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>