Multi-tenant database isolation

Design the database layer for a B2B SaaS serving 50,000 tenants ranging from 5-seat startups to 50,000-seat enterprises, totaling 8 TB and 90k QPS. You must provide strong tenant data isolation, prevent one large tenant from starving others (noisy neighbor), and support per-tenant backup/restore and 'delete all data for tenant X' for compliance. What isolation model and partitioning do you choose?