Phase 3.3 -- Department Health, Provider Status, Activity Feed

Parent: #762

Context

The new dashboard needs department-level health aggregation, provider health status, and a REST-based activity feed (fallback for WebSocket).

Scope

GET /api/v1/departments/{dept_name}/health

• Response: agent_count, active_agent_count, utilization_percent, avg_performance_score, department_cost_7d, cost_trend, collaboration_score
• Data source: aggregate TaskMetricRecord + CollaborationScoreResult by department

Extend GET /api/v1/providers/{name}

• Add fields: health_status (enum: up/degraded/down), last_check_timestamp, avg_response_time_ms, error_rate_percent_24h, calls_last_24h
• Data source: new ProviderHealthRecord model tracking call outcomes + response times

GET /api/v1/activities (paginated)

• Query params: type (filter), agent_id (filter), last_n_hours (24/48/168)
• Response: paginated ActivityLogEntry with event_type, actor, action, target, timestamp, related_ids
• Data source: WebSocket event history + AgentLifecycleEvent + TaskMetricRecord events
• Purpose: REST fallback for non-WebSocket clients, also used for historical activity feeds

Deliverables

• Department health aggregation endpoint
• Extended provider response with health data
• Unified activity feed endpoint
• ProviderHealthRecord model and tracking
• Unit tests
• OpenAPI schema updated