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/dialog/flows/core.py

@@ -66,9 +66,13 @@ def menu_reply(ctx: Ctx, text: str | None = None) -> Reply:
 async def onboarding_entry(ctx: Ctx, after: str | None = None) -> str:
     """Entry state of the next module onboarding: `"{module}.onboarding.start"` for the first module of the
     profile (after `after`, if given) that has such a state registered; `"idle"` when none is left."""
+    from app.core.demo_settings import screening_enabled
+    from app.core.intake_runtime import intake_enabled
+
+    if after is None and not await screening_enabled(ctx.session):
+        return "idle"  # demo switch: no initial questionnaires, straight to the chat
     profile = await ctx.profile()
     modules = list(profile.modules) if profile is not None and profile.modules else []
-    from app.core.intake_runtime import intake_enabled
 
     # Stage 1 is the same for everyone (ADR-012): it does not depend on modules, which it sets itself — an anonymous
     # person has no profile at all before it.