refactor(sofiia-console): extract idempotency store abstraction

Move idempotency TTL/LRU logic into a dedicated store module with a swap-ready interface and wire chat send flow to use store get/set semantics without changing API behavior.

Made-with: Cursor

tests/conftest.py

@@ -28,12 +28,16 @@ def sofiia_module(tmp_path, monkeypatch):
     asyncio.set_event_loop(loop)
 
     import app.db as db_mod  # type: ignore
+    import app.idempotency as idem_mod  # type: ignore
     import app.main as main_mod  # type: ignore
 
     importlib.reload(db_mod)
+    importlib.reload(idem_mod)
     importlib.reload(main_mod)
     main_mod._rate_buckets.clear()
-    main_mod._idempotency_cache.clear()
+    store = idem_mod.get_idempotency_store()
+    if hasattr(store, "reset"):
+        store.reset()
     return main_mod