feat: presentation demo mode — one-click private demo patient, screening on/off switch

- POST /auth/demo/fresh + «Демо: войти как Андрей» on the welcome screen: every sign-in gets a fresh private
  copy of veteran Andrey (consent, prescription, ЕМИАС record) and lands in the chat — no codes, nobody's
  crisis or half-done questionnaire leaks into another viewer.
- Initial screening switch: SCREENING_ENABLED env default + /demo/settings + a toggle in the demo console;
  off → after consent straight to the chat.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

dashboard/console/api.py

@@ -103,6 +103,12 @@ class ConsoleApi:
     def reset(self) -> dict[str, Any]:
         return dict(self._request("POST", "/api/v1/demo/reset", timeout=60))
 
+    def demo_settings(self) -> dict[str, Any]:
+        return dict(self._request("GET", "/api/v1/demo/settings"))
+
+    def set_screening(self, enabled: bool) -> dict[str, Any]:
+        return dict(self._request("POST", "/api/v1/demo/settings", json={"screening_enabled": enabled}))
+
     # ------------------------------------------------------------------ patient dialog
     def send_event(
         self,