System Architecture Explainer

I'm building [SYSTEM DESCRIPTION — e.g. 'a real-time chat application', 'a multi-tenant SaaS platform']. My expected scale: [SCALE — e.g. '10,000 DAU', '100 requests/second']. Current tech stack: [STACK]. Explain the recommended architecture for this system, covering: (1) a high-level architecture diagram described in text (components, how they connect, data flow), (2) the right database choice(s) and why (SQL vs NoSQL, caching layer), (3) key architectural patterns to apply (microservices vs monolith, event-driven, CQRS, etc.) and the tradeoff for each, (4) the 3 biggest scalability risks in this design and how to mitigate them, (5) a phased build plan: what to build in Phase 1 (MVP), Phase 2 (growth), and Phase 3 (scale). Be opinionated and practical.