Register privy/app when Privy appears in Stripe providers index

[snowmead]

Blocker

privy/app is HELD. In the committed catalog fixture, but missing from the live Stripe providers index (Unknown provider).

Unblock

1. Confirm Privy is linkable via stripe projects link privy.
2. Declare pub mod privy + registry row.
3. discover-apply, drop EXCL, restore PROVIDERS.md.

See docs/ADDING-A-PROVIDER.md § External pin blockers.

Related: #90