Add username support for SourceHut

[TedTschopp]

SourceHut (sr.ht) username validation

Category: dev

Goal: Add validation for SourceHut user handles.

Suggested URL format (to verify):

• Candidate profile URL: https://sr.ht/~{username}

Logic (to confirm while implementing):

• Ideally:
  • 200 → username exists
  • 404 → username not found
• If SourceHut behaves differently, document and adapt the logic accordingly.

Implementation notes:

• Add dev/sourcehut.py.
• Implement validate_sourcehut(username: str).
• Prefer status_validate() if status-based; otherwise use generic_validate() and inspect response.text.