Add viewed history, diagnostics, tag chips and SWF viewer

lib/backend/models/provider_diagnostics.dart

@@ -0,0 +1,13 @@
+class ProviderDiagnostics {
+  const ProviderDiagnostics({
+    required this.providerId,
+    required this.lastSearchAt,
+    required this.lastResultCount,
+    this.lastErrorMessage,
+  });
+
+  final String providerId;
+  final DateTime lastSearchAt;
+  final int lastResultCount;
+  final String? lastErrorMessage;
+}

lib/backend/models/viewed_post.dart

@@ -0,0 +1,16 @@
+class ViewedPost {
+  const ViewedPost({
+    required this.viewedKey,
+    required this.providerId,
+    required this.postId,
+    required this.viewedAt,
+  });
+
+  final String viewedKey;
+  final String providerId;
+  final String postId;
+  final DateTime viewedAt;
+
+  static String keyFor(String providerId, String postId) =>
+      '$providerId:$postId';
+}