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>

app/config.py

@@ -78,6 +78,8 @@ class Settings(BaseSettings):
 
     # Virtual clock for the demo (ADR-005).
     demo_clock: bool = True
+    # Initial screening (onboarding questionnaires) on/off — the default; the demo console can flip it at runtime.
+    screening_enabled: bool = True
     tz: str = "Europe/Moscow"
 
     api_base_url: str = "http://localhost:8000"