Add no-op session cache when session cache is disabled

Use NoOpSessionCache in backend/app/main.py and dynamically switch cache implementation in backend/app/dependencies.py so disabled cache mode remains safe while get_session_cache always returns a valid object.

backend/app/dependencies.py

@@ -26,7 +26,7 @@ from app.repositories.protocols import SessionRepository
 from app.services.protocols import AuthService, JailService
 from app.utils.constants import SESSION_COOKIE_NAME
 from app.utils.runtime_state import RuntimeState
-from app.utils.session_cache import SessionCache
+from app.utils.session_cache import InMemorySessionCache, NoOpSessionCache, SessionCache
 
 log: structlog.stdlib.BoundLogger = structlog.get_logger()
 
@@ -79,6 +79,17 @@ def _session_cache_enabled(settings: Settings) -> bool:
 
 def _build_app_context(request: Request) -> ApplicationContext:
     state = cast("AppState", request.app.state)
+    session_cache = getattr(state, "session_cache", None)
+    if session_cache is None:
+        session_cache = NoOpSessionCache()
+        state.session_cache = session_cache
+    elif _session_cache_enabled(state.settings) and isinstance(session_cache, NoOpSessionCache):
+        session_cache = InMemorySessionCache()
+        state.session_cache = session_cache
+    elif not _session_cache_enabled(state.settings) and not isinstance(session_cache, NoOpSessionCache):
+        session_cache = NoOpSessionCache()
+        state.session_cache = session_cache
+
     return ApplicationContext(
         settings=state.settings,
         http_session=getattr(state, "http_session", None),
@@ -88,7 +99,7 @@ def _build_app_context(request: Request) -> ApplicationContext:
         last_activation=getattr(state, "last_activation", None),
         runtime_settings=getattr(state, "runtime_settings", None),
         runtime_state=state.runtime_state,
-        session_cache=getattr(state, "session_cache", None),
+        session_cache=session_cache,
     )