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/api/routes.py

@@ -288,3 +288,22 @@ async def demo_personas() -> list[DemoPersona]:
             )
         )
     return out
+
+
+@router.get("/demo/settings", tags=["demo"])
+async def get_demo_settings() -> dict[str, bool]:
+    """Presentation switches: `screening_enabled` — initial questionnaires after consent (off → straight to chat)."""
+    from app.core import demo_settings
+
+    async with get_clinical_session() as s:
+        return await demo_settings.get_all(s)
+
+
+@router.post("/demo/settings", tags=["demo"])
+async def set_demo_settings(body: dict[str, bool]) -> dict[str, bool]:
+    from app.core import demo_settings
+
+    async with get_clinical_session() as s:
+        if demo_settings.SCREENING in body:
+            await demo_settings.set_screening(s, bool(body[demo_settings.SCREENING]))
+        return await demo_settings.get_all(s)