Stabilize runtime auth and E2E coverage

apps/api/app/runtime_policy.py

@@ -0,0 +1,33 @@
+"""Runtime fallback policy shared by auth/session routes.
+
+The in-process stores exist only to keep local development usable when Docker or
+NAS PostgreSQL is offline. Staging/prod must fail loudly instead of becoming a
+second source of truth.
+"""
+
+from __future__ import annotations
+
+from typing import NoReturn
+
+from fastapi import HTTPException, status
+
+from .config import settings
+
+
+def runtime_fallback_allowed() -> bool:
+    return settings.environment == "dev"
+
+
+def raise_runtime_fallback_disabled(feature: str) -> NoReturn:
+    raise HTTPException(
+        status_code=status.HTTP_503_SERVICE_UNAVAILABLE,
+        detail=(
+            f"{feature} persistence unavailable; runtime fallback is disabled "
+            f"in {settings.environment}"
+        ),
+    )
+
+
+def require_runtime_fallback_allowed(feature: str) -> None:
+    if not runtime_fallback_allowed():
+        raise_runtime_fallback_disabled(feature)