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/utils/session_cache.py

@@ -55,3 +55,19 @@ class InMemorySessionCache:
 
     def clear(self) -> None:
         self._entries.clear()
+
+
+class NoOpSessionCache:
+    """A no-op session cache used when caching is disabled."""
+
+    def get(self, token: str) -> Session | None:
+        return None
+
+    def set(self, token: str, session: Session, ttl_seconds: float) -> None:
+        return None
+
+    def invalidate(self, token: str) -> None:
+        return None
+
+    def clear(self) -> None:
+        return None