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/pages/SurveyPage.tsx

@@ -96,16 +96,14 @@ const SLIDES: SurveySlideData[] = [
   {
     id: 'ranking',
     questions: [
-      { id: 'domain_rank', type: 'rank', num: '14', text: 'Drag to rank: which technical domains should FlowPilot handle first?', hint: 'Most important at the top.',
+      { id: 'prioritization', type: 'rank', num: '14', text: 'Rank these factors by how much they influence your diagnostic order.', hint: 'What drives which theory you investigate first? Drag to reorder — most influential at the top.',
         items: [
-          "Windows Server / Active Directory",
-          "Microsoft 365 / Exchange",
-          "Networking (DNS, DHCP, VPN, Firewall)",
-          "Security / Compliance",
-          "Virtualization (Hyper-V, VMware)",
-          "Backup & Disaster Recovery",
-          "Cloud (Azure / AWS)",
-          "Endpoint Management"
+          "Likelihood of being the root cause",
+          "How fast I can test or rule it out",
+          "Blast radius — impact if it's the problem",
+          "How recently something changed",
+          "Tool / access availability right now",
+          "Past experience with similar symptoms"
         ]},
     ]
   },