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.

Docs/Tasks.md

@@ -667,6 +667,8 @@ A named 16-worker thread pool that is never actually used consumes OS thread res
 
 ### Task 23 — Fix InMemorySessionCache instantiated when disabled
 
+**Status:** Completed
+
 **Severity:** Low
 
 **Where:**