feat: add Tube8 email scan module

[brunolm]

What

Adds an email-based Tube8 account check to email_scan/adult/.

How it works

Tube8 is on the same Aylo/MindGeek backend as Pornhub / RedTube / YouPorn and uses the identical signup validation flow:

1. GET /register to scrape the per-session page_params.token (ends in a literal . that must be preserved).
2. POST /register/verify_email with token + email.

Response mapping:

Response	Result
{"success":true}	Not Registered
{"success":false, ["...does not meet our registration requirements."]} (valid email)	Registered
{"success":false, ["Not available."]} (stale/over-used token throttle)	Error (rate limited)
non-200 / 429	Error

For a well-formed address, Tube8 reports an existing account by refusing registration ("does not meet our registration requirements") rather than saying it's taken — same behavior as YouPorn/RedTube. The is_valid_email guard ensures malformed inputs aren't misreported as hits.

Shared helper

Reuses the is_valid_email helper in core/helpers.py. This is the same helper introduced alongside RedTube (PR #417) and YouPorn (PR #420) — byte-for-byte identical — so overlapping merges resolve as a no-op on helpers.py.

Notes

• Email-only: Tube8 user profiles are addressed by numeric ID (Aylo platform), so username lookups by name aren't viable.
• Neither holehe nor MailAccess has a Tube8 checker.
• The module is a domain-swap of the YouPorn module.

Testing

• Verified live: test@gmail.com → Registered; throwaway addresses → Not Registered.
• pytest tests/: all pass except 2 pre-existing Windows chmod(0) no-op tests, unrelated to this change.

Note: like the other Aylo sites, the endpoint's anti-enumeration throttle can intermittently return the "requirements" message for a valid address, so a Registered result on a valid email is high-signal but not a hard guarantee; Not Registered is reliable.

[kaifcodec]

LGTM.