Scaffold backend gateway and integration docs

lib/core/auth/in_memory_token_store.dart

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+import 'package:relationship_saver/core/auth/token_store.dart';
+import 'package:relationship_saver/integrations/backend/models/backend_models.dart';
+
+/// Simple in-memory [TokenStore] used in tests and local fakes.
+class InMemoryTokenStore implements TokenStore {
+  AuthSession? _session;
+
+  @override
+  Future<void> clear() async {
+    _session = null;
+  }
+
+  @override
+  Future<AuthSession?> read() async => _session;
+
+  @override
+  Future<void> write(AuthSession session) async {
+    _session = session;
+  }
+}

lib/core/auth/secure_token_store.dart

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+import 'dart:convert';
+
+import 'package:flutter_secure_storage/flutter_secure_storage.dart';
+import 'package:relationship_saver/core/auth/token_store.dart';
+import 'package:relationship_saver/integrations/backend/models/backend_models.dart';
+
+/// [TokenStore] backed by platform secure storage.
+class SecureTokenStore implements TokenStore {
+  SecureTokenStore({FlutterSecureStorage? storage})
+    : _storage = storage ?? const FlutterSecureStorage();
+
+  static const String _sessionKey = 'backend_auth_session_v1';
+  final FlutterSecureStorage _storage;
+
+  @override
+  Future<AuthSession?> read() async {
+    final String? raw = await _storage.read(key: _sessionKey);
+    if (raw == null || raw.isEmpty) {
+      return null;
+    }
+
+    try {
+      final Map<String, dynamic> json = jsonDecode(raw) as Map<String, dynamic>;
+      return AuthSession.fromJson(json);
+    } on FormatException {
+      await clear();
+      return null;
+    }
+  }
+
+  @override
+  Future<void> write(AuthSession session) async {
+    final String raw = jsonEncode(session.toJson());
+    await _storage.write(key: _sessionKey, value: raw);
+  }
+
+  @override
+  Future<void> clear() => _storage.delete(key: _sessionKey);
+}

lib/core/auth/token_store.dart

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+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();
+}