feat(admin): AI quality preview showing per-(severity x provider) helpful-rate

v1.4.16 phase B5e: ship the `/admin/ai-quality` route that surfaces
the daily aggregator's helpful-rate per (severity x metricSourceType
x provider x prompt-version) bucket. Renders as a compact 7-column
table with severity tinted by clinical-importance colour and the
helpful-rate column tinted green/yellow/orange by 0.8/0.5
thresholds.

New endpoint `GET /api/admin/ai-quality` (requireAdmin-gated) returns
the raw `AppSettings.adminAiInsightsFeedbackSummary` blob so the
v1.4.17 ratchet that consumes the same shape for prompt-tuning can
read what the worker writes without an envelope-reshape step.

Empty state ("no summary yet" / "buckets[] is empty") rendered as a
quiet message rather than a fake zero row so the operator can tell
"no feedback" apart from "all-zeros". Loading + error banners follow
the existing host-metrics pattern.

Section wiring:
  - new `ai-quality` slug in ADMIN_SECTION_SLUGS (between
    integrations and feedback so the IA flows General → Integrations
    → AI Quality → Feedback Inbox)
  - new Sparkles icon entry in admin-shell
  - new switch case in [section]/renderer.tsx
  - EN+DE i18n keys: admin.section.ai-quality.{title,subtitle} +
    admin.aiQuality.{title,loading,loadError,empty,windowLabel,
    generatedAtLabel,col*}

Tests:
  - 5 new ai-quality-section tests (loading, empty-null, empty-buckets,
    table-render with multiple buckets, error)
  - 1 admin-sections smoke test ensures the empty branch renders +
    1 slug-list parity test

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/components/admin/admin-shell.tsx

@@ -28,6 +28,7 @@ import {
   Server,
   Settings,
   ShieldAlert,
+  Sparkles,
   Users,
   type LucideIcon,
 } from "lucide-react";
@@ -69,6 +70,11 @@ export const ADMIN_SECTIONS: readonly AdminSection[] = [
     titleKey: "admin.section.integrations.title",
     icon: AlertTriangle,
   },
+  {
+    slug: "ai-quality",
+    titleKey: "admin.section.ai-quality.title",
+    icon: Sparkles,
+  },
   {
     slug: "feedback",
     titleKey: "admin.section.feedback.title",