handle server-side deletions (#5)

There is no obvious way to find the needle in the haystack.
The adopted strategy is to fetch entries metadata in big bulks. While it means
fetching the whole referential it also allow to get the most out of the least
round-trip.

lib/main.dart

@@ -31,12 +31,12 @@ final _log = Logger('frigoligo');
 // - [x] better server configuration flow (more interactive)
 // - [x] save scroll position in article view
 // - [x] padding and spacing everywhere
-// - [ ] handle server side deletion of articles
+// - [x] handle server side deletion of articles
 // - [ ] unit tests (at least for WallabagClient) -> https://github.com/wallabag/docker#sqlite
-// - [ ] handle filtering by tag
 // 0.3
 // - [ ] add article (in app + share sheet)
 // - [ ] translations -> https://docs.flutter.dev/accessibility-and-localization/internationalization
+// - [ ] handle filtering by tag
 // 1.0
 // - [ ] background sync
 // - [ ] notification badge with https://pub.dev/packages/flutter_app_badger ?

lib/services/wallabag.dart

@@ -112,6 +112,30 @@ class ArticlesProvider with ChangeNotifier {
   int count(StateFilter state, StarredFilter starred) =>
       _buildQuery(state: state, starred: starred).countSync();
 
+  Future<int> syncRemoteDeletes() async {
+    final remoteCount = await wallabag.fetchTotalEntriesCount();
+    var localIds = (await db.articles.where().idProperty().findAll()).toSet();
+    final delta = localIds.length - remoteCount;
+    if (delta <= 0) return 0;
+    _log.info('server-side deletion detected: delta=$delta');
+
+    // seems overkill but the only way to enumerate all entry ids efficiently
+    final entriesStream = wallabag.fetchAllEntries(
+      perPage: 100,
+      detail: DetailValue.metadata,
+    );
+    await for (final (entries, _) in entriesStream) {
+      localIds = localIds.difference(entries.map((e) => e.id).toSet());
+    }
+
+    await db.writeTxn(() async {
+      await db.articles.deleteAll(localIds.toList());
+      await db.articleScrollPositions.deleteAll(localIds.toList());
+    });
+
+    return delta;
+  }
+
   Future<int> fullRefresh({int? since}) async {
     assert(!refreshInProgress);
 
@@ -151,6 +175,7 @@ class ArticlesProvider with ChangeNotifier {
     }
     _log.info(
         'completed refresh of $count entries in ${stopwatch.elapsed.inSeconds} s');
+    syncRemoteDeletes();
 
     final now = DateTime.now().millisecondsSinceEpoch / 1000;
     await SharedPreferences.getInstance()