Design a multi-tenant architecture

Design a multi-tenant data architecture for [SaaS Application]. Systematically compare three tenancy models: a separate database per tenant for maximum isolation, a separate schema per tenant within a shared database, and a shared schema with a tenant_id column on every table. Evaluate each model against your requirements for data isolation, operational complexity, cross-tenant querying needs, and projected scale. Recommend the best fit, describe the data access layer that enforces tenant isolation at the application level, and explain the onboarding process for adding a new tenant.