Scaffold backend gateway and integration docs

lib/integrations/backend/backend_gateway.dart

@@ -0,0 +1,47 @@
+import 'package:relationship_saver/integrations/backend/models/backend_models.dart';
+
+/// Domain-level backend boundary independent of HTTP transport.
+abstract interface class BackendGateway {
+  /// Authenticates user and returns active session.
+  Future<AuthSession> signIn(SignInRequest request);
+
+  /// Signs out current user.
+  Future<void> signOut();
+
+  /// Returns current user profile.
+  Future<UserProfile> me();
+
+  /// Pulls remote changes since optional cursor.
+  Future<SyncPullResult> pullChanges({String? cursor, int limit = 200});
+
+  /// Pushes local changes to backend.
+  Future<SyncPushResult> pushChanges({
+    required List<ChangeEnvelope> changes,
+    String? cursor,
+  });
+
+  /// Fetches recommendation signals feed.
+  Future<SignalsFeed> getSignalsFeed({
+    String? cursor,
+    DateTime? since,
+    int limit = 50,
+    Set<String>? personIds,
+  });
+
+  /// Acknowledges a signal action.
+  Future<void> acknowledgeSignal({
+    required String signalId,
+    required SignalAction action,
+    DateTime? at,
+  });
+
+  /// Initializes upload metadata and pre-signed URL.
+  Future<UploadInitResult> initUpload({
+    required UploadPurpose purpose,
+    required String contentType,
+    required int sizeBytes,
+  });
+
+  /// Resolves a temporary download URL.
+  Future<DownloadUrl> getDownloadUrl({required String fileId});
+}