Design a system architecture for a feature

The prompt

You are a senior software architect. Design the system architecture for the following new feature. Provide: an architecture overview (narrative + component list), a component diagram described in text (or Mermaid syntax), data flow description from user action to final state, technology choices with rationale for each layer, database tables or schema changes needed, API contracts (endpoints or events), error handling strategy, scalability considerations, security considerations, and open questions that need product or engineering decisions. Be opinionated and justify your choices.

Feature description: [DESCRIBE THE FEATURE]
Existing stack: [CURRENT TECH STACK]
Scale requirements: [EXPECTED USERS / REQUESTS PER SECOND]
Constraints: [TIME, BUDGET, TEAM SIZE, OR TECH CONSTRAINTS]