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/public/mod.rs

@@ -8,10 +8,12 @@ pub(crate) use self::ai_public::{
 };
 pub(crate) use self::catalog_helpers::{
     admin_requested_force_stream, api_format_display_name, build_api_format_health_monitor_payload,
+    build_model_health_monitor_payload, build_provider_health_monitor_payload,
     build_public_catalog_models_payload, build_public_catalog_search_models_payload,
     build_public_health_timeline, build_public_providers_payload, normalize_admin_base_url,
     provider_key_api_formats, request_candidate_event_unix_ms, request_candidate_status_label,
     sanitize_public_model_config_for_user, ApiFormatHealthMonitorOptions,
+    ModelHealthMonitorOptions,
 };
 pub(crate) use self::system_modules_helpers::{
     build_admin_keys_grouped_by_format_payload, build_public_auth_modules_status_payload,