fix: survey invite links use FRONTEND_URL config instead of hardcoded URL

PR environments were generating survey links pointing to production
(resolutionflow.com) because the URL was hardcoded. Now uses the
existing settings.FRONTEND_URL, falling back to localhost (debug)
or production (release).

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

backend/app/api/endpoints/admin_survey.py

@@ -29,11 +29,14 @@ logger = logging.getLogger(__name__)
 
 router = APIRouter(prefix="/admin", tags=["admin-survey"])
 
-FRONTEND_URL = "https://resolutionflow.com"
+def _get_frontend_url() -> str:
+    if settings.FRONTEND_URL:
+        return settings.FRONTEND_URL
+    return "http://localhost:5173" if settings.DEBUG else "https://resolutionflow.com"
 
 
 def _build_invite_response(invite: SurveyInvite) -> SurveyInviteResponse:
-    base_url = FRONTEND_URL if not settings.DEBUG else "http://localhost:5173"
+    base_url = _get_frontend_url()
     return SurveyInviteResponse(
         id=str(invite.id),
         token=invite.token,
@@ -63,7 +66,7 @@ async def create_survey_invite(
 
     if data.send_email and data.recipient_email:
         try:
-            base_url = FRONTEND_URL if not settings.DEBUG else "http://localhost:5173"
+            base_url = _get_frontend_url()
             survey_url = f"{base_url}/survey?t={invite.token}"
             sent = await EmailService.send_survey_invite_email(
                 to_email=data.recipient_email,