Problem

AgentActivityEvent.event_type is typed as string, but the codebase uses a finite, known set of event type values in ACTIVITY_ICON_MAP (utils/agents.ts). This means key typos are invisible to the compiler.

In contrast, CareerEvent.event_type correctly uses the CareerEventType literal union -- the inconsistency reduces type safety.

Proposed fix

1. Define ActivityEventType literal union in web/src/api/types.ts with the known values: hired, fired, promoted, demoted, onboarded, task_completed, task_started, cost_incurred, tool_used, delegation_sent, delegation_received
2. Use it in AgentActivityEvent.event_type (with | (string & {}) fallback for unknown future types)
3. Update ACTIVITY_ICON_MAP to use Record<ActivityEventType, LucideIcon>

Found during PR #874 review (type-design-analyzer agent).