Add username support for Quora

[TedTschopp]

Quora username validation

Category: social

Goal: Add a validator for Quora profile handles.

Suggested URL format (to verify):

• Candidate profile URL: https://www.quora.com/profile/{username}

Logic (to confirm):

• If 200 vs 404 is reliable, use status_validate().
• Otherwise parse HTML with generic_validate() to distinguish between existing and non-existing profiles.

Implementation notes:

• Add social/quora.py with validate_quora(username: str).
• Follow existing patterns for headers and error handling.