feat: add Vimeo email scan module

[brunolm]

Adds an email-existence scan module for Vimeo.

How it works

• Fetches the xsrft token from GET https://vimeo.com/_next/viewer, then POST https://vimeo.com/join with email_validation=true.
• This is a validation-only call: no account is created, there is no captcha on this step, and no email is sent.

Signal

• Registered → HTTP 418 {"has_error_user_exists": true} → Result.taken
• Not registered → HTTP 200 [] → Result.available

Testing

• Verified live end-to-end: a registered address returns Registered, a random unregistered address returns Not Registered.
• Auto-discovered (no registry changes). Full test suite passes (the only failures are the pre-existing *_unreadable tests that no-op on Windows).

[kaifcodec]

LGTM.