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/core/demo_settings.py

@@ -0,0 +1,34 @@
+"""Presentation switches (demo): stored in `demo_settings`, default from settings/env.
+
+    SCREENING_ENABLED=false                 # env default
+    POST /api/v1/demo/settings {"screening_enabled": false}   # one button in the demo console
+
+Screening off: after consent the patient goes straight to the chat — no onboarding questionnaires.
+"""
+
+from typing import Any
+
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from app.config import get_settings
+from app.db.clinical import DemoSetting
+
+SCREENING = "screening_enabled"
+
+
+async def get_all(session: AsyncSession) -> dict[str, Any]:
+    row = await session.get(DemoSetting, SCREENING)
+    return {SCREENING: bool(row.value) if row is not None else get_settings().screening_enabled}
+
+
+async def screening_enabled(session: AsyncSession) -> bool:
+    return (await get_all(session))[SCREENING]
+
+
+async def set_screening(session: AsyncSession, enabled: bool) -> None:
+    row = await session.get(DemoSetting, SCREENING)
+    if row is None:
+        session.add(DemoSetting(key=SCREENING, value=enabled))
+    else:
+        row.value = enabled
+    await session.flush()