Multi-tenant search platform

Design a multi-tenant full-text search platform for an e-commerce SaaS where 200k merchant stores share the cluster. Catalog sizes range from 50 products (long tail) to 8M products (a few whales). Search must return in under 100ms p95, indexing of a product edit must be visible within 5s, and one noisy whale must not degrade a small merchant's latency. Read:write is roughly 100:1.