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/integrations/emias_record.py

@@ -77,7 +77,11 @@ class EmiasRecordSource:
 
     async def get(self, token: str) -> PatientRecord | None:
         try:
-            base = _load(self._path or default_records_path()).get(token)
+            records = _load(self._path or default_records_path())
+            base = records.get(token)
+            if base is None and "-" in token:
+                # Fresh demo copies (POST /auth/demo/fresh): "tok-andrey-1a2b3c4d" → the "tok-andrey" record.
+                base = records.get(token.rsplit("-", 1)[0])
         except (OSError, yaml.YAMLError, ValueError) as exc:
             log.error("emias records unavailable: %s", type(exc).__name__)
             base = None