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/constants.rs

@@ -93,6 +93,7 @@ pub(crate) const RUST_FRONTDOOR_OWNED_ROUTE_PATTERNS: &[&str] = &[
     "/api/public/stats",
     "/api/public/global-models",
     "/api/public/health/api-formats",
+    "/api/public/health/models",
     "/api/oauth/providers",
     "/api/oauth/{provider_type}/authorize",
     "/api/oauth/{provider_type}/callback",