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/db/clinical.py

@@ -192,3 +192,12 @@ class ClockState(ClinicalBase):
 
     id: Mapped[int] = mapped_column(Integer, primary_key=True)
     offset_seconds: Mapped[float] = mapped_column(Float, default=0.0)
+
+
+class DemoSetting(ClinicalBase):
+    """Presentation switches changed at runtime from the demo console (e.g. the initial screening on/off)."""
+
+    __tablename__ = "demo_settings"
+
+    key: Mapped[str] = mapped_column(String(64), primary_key=True)
+    value: Mapped[Any] = mapped_column(JSONType)