feat(health): add model and provider health monitoring

- Rename the original health monitor to endpoint health monitor
- Add tab navigation for endpoint, model, and provider health views
- Add model health monitor cards with availability, latency, first-byte latency, and 60-point history
- Add admin-only provider health monitor with collapsible active-provider sections
- Show per-provider model health cards after expanding a provider
- Add backend model health and provider health monitor payload builders
- Add admin endpoint for provider health monitoring
- Add provider-scoped usage breakdown filtering for per-provider model statistics
- Add frontend API types and request helpers for model/provider health data
- Add demo mock data for model and provider health monitoring
- Fix model health timeline time-unit handling so request history segments render correctly

Verification:
- cargo fmt
- npm run type-check
- npm run build
- cargo test -p aether-gateway health_models
- cargo test -p aether-gateway health_providers
- cargo test -p aether-gateway gateway_exposes_frontdoor_manifest_without_proxying_upstream

apps/aether-gateway/src/handlers/admin/request/observability.rs

@@ -242,6 +242,40 @@ impl<'a> AdminAppState<'a> {
         .await
     }
 
+    pub(crate) async fn build_model_health_monitor_payload(
+        &self,
+        lookback_hours: u64,
+        model_limit: usize,
+        per_model_limit: usize,
+        options: crate::handlers::public::ModelHealthMonitorOptions,
+    ) -> Option<serde_json::Value> {
+        crate::handlers::public::build_model_health_monitor_payload(
+            self.app,
+            lookback_hours,
+            model_limit,
+            per_model_limit,
+            options,
+        )
+        .await
+    }
+
+    pub(crate) async fn build_provider_health_monitor_payload(
+        &self,
+        lookback_hours: u64,
+        provider_limit: usize,
+        per_provider_model_limit: usize,
+        per_model_event_limit: usize,
+    ) -> Option<serde_json::Value> {
+        crate::handlers::public::build_provider_health_monitor_payload(
+            self.app,
+            lookback_hours,
+            provider_limit,
+            per_provider_model_limit,
+            per_model_event_limit,
+        )
+        .await
+    }
+
     pub(crate) async fn execute_execution_runtime_sync_plan(
         &self,
         trace_id: Option<&str>,