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

crates/aether-data/src/repository/usage/memory.rs

@@ -528,6 +528,11 @@ fn usage_matches_breakdown_summary_query(
             return false;
         }
     }
+    if let Some(provider_name) = query.provider_name.as_deref() {
+        if item.provider_name != provider_name {
+            return false;
+        }
+    }
     match query.group_by {
         UsageBreakdownGroupBy::Model | UsageBreakdownGroupBy::Provider => true,
         UsageBreakdownGroupBy::ApiFormat => item.api_format.is_some(),