feat: admin survey responses page with expandable detail and CSV export

- Backend: GET /admin/survey-responses (list with stats, invite join)
- Backend: GET /admin/survey-responses/export (CSV download)
- Frontend: SurveyResponsesPage with expandable row detail
- Two-column Q&A grid with typed answer rendering (chips, ranked lists, quote blocks)
- Stats cards (total responses, this week)
- CSV export button with blob download
- Sidebar nav + route wiring
- Also: updated Q14 from product domain ranking to diagnostic prioritization

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/components/admin/AdminSidebar.tsx

@@ -9,6 +9,7 @@ import {
   Settings,
   FolderTree,
   ClipboardList,
+  MessageSquareText,
   ArrowLeft,
 } from 'lucide-react'
 import { cn } from '@/lib/utils'
@@ -23,6 +24,7 @@ const navItems = [
   { path: '/admin/settings', label: 'Settings', icon: Settings },
   { path: '/admin/categories', label: 'Categories', icon: FolderTree },
   { path: '/admin/survey-invites', label: 'Survey Invites', icon: ClipboardList },
+  { path: '/admin/survey-responses', label: 'Survey Responses', icon: MessageSquareText },
 ]
 
 interface AdminSidebarProps {