Refactor Prisma schema for multi-tenancy ahead of our first enterprise deal

A signed enterprise customer is forcing a problem we deferred for two years: our SaaS (HR onboarding tooling, ~200 SMB customers) has no real tenant isolation. Everything lives in one PostgreSQL database with a company_id column sprinkled inconsistently across 40-odd Prisma models, and at least three tables are missing it entirely. The engagement is a schema audit, a written migration plan we can defend to the customer's security team, and then the migration itself: consistent tenant scoping, row-level security policies in Postgres, Prisma client middleware that makes cross-tenant queries impossible to write by accident, and a zero-downtime rollout. We have staging infrastructure that mirrors production and full freedom to rehearse there. This needs someone who has done multi-tenant migrations before, not someone who wants to learn on our data. Ask us hard questions in the interview - we will do the same.