refactor(internal): split db/schema.ts into domain files (Commit 3.2)

- Create schema/enums.ts for all pgEnum definitions
- Create schema/users.ts for user-related tables
- Create schema/billing.ts for billing tables
- Create schema/organizations.ts for org tables
- Create schema/agents.ts for agent tables
- Create schema/index.ts for unified re-exports
- Update schema.ts to re-export from schema/

Author: brandonkachen