Register heygen/api when live Stripe catalog lists HeyGen

[snowmead]

Blocker

heygen/api is HELD. May appear linked in status, but live catalog/add returns unknown provider/service.

Unblock

1. Confirm HeyGen is in the live Stripe catalog and add heygen/api works.
2. Declare pub mod heygen + registry row.
3. discover-apply, drop EXCL, restore PROVIDERS.md.

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

Related: #90