Introduce a configurable captcha manager with drivers for image,
Cloudflare Turnstile, and Google reCAPTCHA, including fallback
behaviour.
Refactor login, signup, complain, and related flows to use the new
abstraction while simplifying the legacy image endpoint.
Document captcha environment options and restore classic defaults in
.env.example.
Signed-off-by: Qi HU <github@spcsky.com>
