Scaffold backend gateway and integration docs

lib/core/auth/token_store.dart

@@ -0,0 +1,13 @@
+import 'package:relationship_saver/integrations/backend/models/backend_models.dart';
+
+/// Session token persistence abstraction.
+abstract interface class TokenStore {
+  /// Reads the current session, if any.
+  Future<AuthSession?> read();
+
+  /// Persists a full auth session.
+  Future<void> write(AuthSession session);
+
+  /// Clears all stored auth state.
+  Future<void> clear();
+}