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>

backend/app/schemas/survey.py

@@ -44,3 +44,20 @@ class SurveyInviteStatus(BaseModel):
     """Public invite status check — minimal info."""
     name: str
     status: str
+
+
+class SurveyResponseDetail(BaseModel):
+    """Full survey response returned to admin."""
+    id: str
+    respondent_name: Optional[str]
+    responses: dict[str, Any]
+    source: str
+    invite_name: Optional[str]
+    created_at: datetime
+
+
+class SurveyResponseListResponse(BaseModel):
+    """List of survey responses with summary stats."""
+    responses: list[SurveyResponseDetail]
+    total: int
+    this_week: int