Add PrivatePay entry to registry.json

[noisyboy08]

Private payments for freelancers and individuals: shielded transfers and selective payment disclosure powered by STRK20.

What are you building?

A private payment solution for freelancers and businesses using STRK20 shielded transfers and selective disclosures.

Who's building it?

noisyboy08 (GitHub: https://github.com/noisyboy08)

Checklist

• I added one object to 'registry.json' and did not modify anyone else's entry
• 'slug' is lowercase, hyphenated, and not already taken
• 'repo_url' is public
• 'telegram' has a bare username for everyone on the team

[github-actions]

registry.json does not validate, so this is not going in yet. Every problem is listed in the Registrations run - push a fix to this branch and it will be applied on its own.