feat: check every subscription in the background and add the posts page

app/test/fake_fetch.dart

@@ -0,0 +1,45 @@
+import 'package:actors/actors.dart';
+
+import 'package:mews/application/actors/fetch_actor.dart';
+import 'package:mews/mews/parser.dart';
+
+/// Serves pages from an in-memory map without network or isolates, so
+/// navigation tests are deterministic. [sources] wins over the cached source
+/// a caller offers, the way a server answering 200 does, so a test can change
+/// what a site publishes between two fetches. An address with no source is
+/// unreachable, and the cached copy stands in for it if there is one.
+class FakeFetch implements Actor<FetchCommand, FetchResult> {
+  FakeFetch(this.sources);
+
+  final Map<String, String> sources;
+
+  @override
+  Future<FetchResult> send(FetchCommand message) async {
+    return switch (message) {
+      ParseSource(:final url, :final source) => _parse(url, source),
+      FetchPage(:final url, :final cachedSource) => switch ((
+        sources[url],
+        cachedSource,
+      )) {
+        (final String fresh, _) => _parse(url, fresh),
+        (null, final String cached) => _parse(url, cached),
+        _ => const FetchNetworkError(),
+      },
+      _ => const FetchNetworkError(),
+    };
+  }
+
+  FetchResult _parse(String url, String source) {
+    try {
+      return FetchPageOk(page: parseMewsPage(source: source, url: url), source: source);
+    } on MewsParseException catch (e) {
+      return FetchParseError(e.message, source);
+    }
+  }
+
+  @override
+  Future<void> close() async {}
+
+  @override
+  dynamic noSuchMethod(Invocation invocation) => super.noSuchMethod(invocation);
+}