feat: define realtime and error contracts

fixtures/v1/error.unauthorized.json

@@ -0,0 +1,5 @@
+{
+  "code": "unauthorized",
+  "message": "Authentication is required.",
+  "request_id": "req_fixture_unauthorized"
+}

fixtures/v1/realtime.activity.json

@@ -0,0 +1,9 @@
+{
+  "event_id": "evt_fixture_activity_001",
+  "event_type": "activity.follow",
+  "occurred_at": "2026-01-01T00:00:00Z",
+  "payload": {
+    "username": "fixture_viewer",
+    "display_name": "Fixture Viewer"
+  }
+}