feat: add Disqus email scan module

[brunolm]

Adds an email-existence scan module for Disqus.

How it works

• GET https://disqus.com/profile/signup/ to grab the csrfmiddlewaretoken, then POST the same URL with the email, a blank password, and the required consent checkboxes.
• The blank password makes the form always fail validation, so no account is ever created. Django surfaces the email-uniqueness error before the reCAPTCHA check, so the signal is reachable unauthenticated (no captcha token needed).

Signal

• Registered → body contains The e-mail address you specified is already in use → Result.taken
• Not registered → marker absent but the form still returns an errorlist (the blank-password error, confirming the form ran) → Result.available
• Neither → Result.error (guards against a block page / redesign silently reading as 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 (only the pre-existing *_unreadable tests fail, and only on Windows).

[kaifcodec]

LGTM.