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

Splits the monolithic db/schema.ts into domain-specific schema files:
- schema/users.ts: User-related tables
- schema/organizations.ts: Org-related tables
- schema/billing.ts: Billing and credit tables
- schema/runs.ts: Run and agent tables
- schema/enums.ts: Shared enums
- schema/index.ts: Re-exports all schemas

Fixes: Restores .notNull() constraints on org auto_topup_threshold and auto_topup_amount fields that were accidentally dropped during extraction.

Author: brandonkachen