Replace process-local session cache with pluggable session cache backend

Docs/Tasks.md

@@ -27,6 +27,7 @@ Reference: `Docs/Refactoring.md` for full analysis of each issue.
    - Issue: `_session_cache` in `app/dependencies.py` is a process-local dict, so logout invalidation and session revocation only work within a single process.
    - Propose: Define a cache interface and provide a default in-memory implementation, with the option to swap in shared cache storage (Redis, Memcached) for clustered production deployments.
    - Test: Add unit tests for the cache abstraction and verify logout/invalidation behaves correctly through the configured cache implementation.
+   - Status: completed
 
 4. Harden SQLite connection configuration and lifecycle
    - Goal: Make application database access robust under concurrent requests and background task load.