feat(admin): assistant feature-flag toggle panel

New admin slug `assistant` between `ai-quality` (provider config)
and `coach-feedback` (in-app feedback inbox). The panel renders six
`<SettingsToggle>` rows — master plus five sub-flags — and writes
through a dedicated `PUT /api/admin/settings/assistant-flags`
endpoint. Optimistic UI flips on click; failure surfaces a toast
and re-invalidates the read query.

The dedicated endpoint (separate from the generic `/api/admin/settings`
PUT) lets the panel scope its audit trail to a single
`admin.settings.assistant-flags.update` action and lets the
optimistic UI bind 1:1 to a focused request/response shape. The
PUT handler echoes both the raw column values and the resolved
shape so the panel can render the master vs sub-flag distinction
visually.

Sub-toggles grey out when the master is off (UX-wise) so the
operator can see which sub-flags are individually flipped before
unmuting the master. Writes invalidate the runtime
`/api/feature-flags` cache so the operator sees the toggled
surface react within the same session.

i18n: `admin.section.assistant.{title,subtitle}` and
`admin.assistant.*` keys land in all six locales (DE proper, the
five English-leaning admin locales mirror the English copy as is
the established convention for admin strings).

src/app/admin/[section]/renderer.tsx

@@ -17,6 +17,7 @@ import type { JSX } from "react";
 import { useAuth } from "@/hooks/use-auth";
 import { useTranslations } from "@/lib/i18n/context";
 import { AiQualitySection } from "@/components/admin/ai-quality-section";
+import { AssistantSection } from "@/components/admin/assistant-section";
 import { CoachFeedbackSection } from "@/components/admin/coach-feedback-section";
 import { ApiTokenOverviewSection } from "@/components/admin/api-token-overview-section";
 import { AppLogPreviewSection } from "@/components/admin/app-log-preview-section";
@@ -94,6 +95,15 @@ export function AdminSectionRenderer({
           <AiQualitySection />
         </SectionFrame>
       );
+    case "assistant":
+      return (
+        <SectionFrame
+          title={t("admin.section.assistant.title")}
+          subtitle={t("admin.section.assistant.subtitle")}
+        >
+          <AssistantSection />
+        </SectionFrame>
+      );
     case "coach-feedback":
       return (
         <SectionFrame