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

frontend/src/features/providers/components/HealthMonitorCard.vue

@@ -6,9 +6,14 @@
     <!-- 标题和筛选器 -->
     <div class="px-6 py-3.5 border-b border-border/60">
       <div class="flex items-center justify-between gap-4">
-        <h3 class="text-base font-semibold">
-          {{ title }}
-        </h3>
+        <div>
+          <h3 class="text-base font-semibold">
+            {{ title }}
+          </h3>
+          <p class="mt-1 text-xs text-muted-foreground">
+            基于真实请求统计端点可用率、请求成功率与健康历史
+          </p>
+        </div>
         <div class="flex items-center gap-3">
           <Label class="text-xs text-muted-foreground">回溯时间：</Label>
           <Select