feat: add admin billing management for teams/orgs

Add customer lookup by team/org ID with controls to:
- Change billing mode (SEATS / ACTIVE_USERS)
- Update subscription price per seat (creates new Stripe price)
- Assign minimum seats (paidSeats)

Includes tRPC admin procedures, Stripe integration, and i18n strings.

Co-Authored-By: sean@cal.com <Sean@brydon.io>