Register chroma/database when live Stripe catalog lists Chroma

[snowmead]

Blocker

chroma/database is HELD. Committed catalog fixture includes Chroma, but live:

stripe projects catalog chroma → Unknown provider or category: chroma

Unblock

1. Confirm Chroma appears in the live Stripe Projects catalog.
2. Declare pub mod chroma + registry row.
3. discover-apply, drop EXCL, restore PROVIDERS.md.

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

Related: #90