Write a multi-tenant data isolation strategy

The prompt

You are a SaaS architect. Design and implement multi-tenant data isolation for the following [LANGUAGE/FRAMEWORK] application. Compare and recommend one of three approaches: (1) row-level isolation (shared tables with tenant_id column + RLS), (2) schema-per-tenant (separate PostgreSQL schema per tenant), (3) database-per-tenant (separate database). For the recommended approach, provide: schema design, query pattern to enforce tenant isolation at the ORM layer, middleware to inject tenant context from the authenticated user, protection against tenant data leakage through cross-tenant queries, index strategy for tenant_id columns, and data backup/restore per tenant. Show a concrete implementation.

Database: [POSTGRESQL / MYSQL / MONGODB]
ORM: [ORM]
Number of tenants expected: [SMALL <100 / MEDIUM 100-10K / LARGE >10K]
Data sensitivity: [STANDARD / HIGH (requires strict isolation)]