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

app/lib/database/database.dart

@@ -286,6 +286,51 @@ class MewsDatabase extends _$MewsDatabase {
     return statement.write(const FeedEntriesCompanion(readAt: Value(null)));
   }
 
+  /// Every stored post with the site it came from, for the posts page.
+  Future<List<RiverEntry>> riverEntries({
+    EntrySort sort = EntrySort.date,
+    bool unreadOnly = false,
+    int limit = 200,
+  }) async {
+    final site = alias(subscriptions, 'site');
+    final query = select(feedEntries).join([
+      leftOuterJoin(site, site.url.equalsExp(feedEntries.siteUrl)),
+    ]);
+    if (unreadOnly) query.where(feedEntries.readAt.isNull());
+
+    // A site without a title sorts under its address, so the order never
+    // depends on a null.
+    final sourceName = coalesce([site.title, feedEntries.siteUrl]);
+    query.orderBy(switch (sort) {
+      EntrySort.date => [
+          OrderingTerm.desc(feedEntries.date),
+          OrderingTerm.asc(feedEntries.url),
+        ],
+      EntrySort.source => [
+          OrderingTerm.asc(sourceName),
+          OrderingTerm.desc(feedEntries.date),
+        ],
+      EntrySort.unread => [
+          // Unread first, whichever way the database orders nulls.
+          OrderingTerm.desc(feedEntries.readAt.isNull()),
+          OrderingTerm.desc(feedEntries.date),
+        ],
+    });
+    query.limit(limit);
+
+    return (await query.get()).map((row) {
+      final entry = row.readTable(feedEntries);
+      return RiverEntry(
+        siteUrl: entry.siteUrl,
+        siteTitle: row.readTableOrNull(site)?.title,
+        url: entry.url,
+        title: entry.title,
+        date: entry.date,
+        readAt: entry.readAt,
+      );
+    }).toList();
+  }
+
   /// How many posts are unread, for one site or across all of them.
   Future<int> unreadCount({String? siteUrl}) async {
     final total = feedEntries.url.count();
@@ -371,3 +416,43 @@ class MewsDatabase extends _$MewsDatabase {
   Future<void> saveCachedPage(CachedPagesCompanion entry) =>
       into(cachedPages).insert(entry, mode: InsertMode.insertOrReplace);
 }
+
+/// How the posts page is ordered.
+enum EntrySort {
+  date,
+  source,
+  unread;
+
+  /// Reads the order from an address, falling back to the default rather than
+  /// refusing a value the reader may have typed.
+  static EntrySort parse(String? name) => switch (name) {
+    'source' => EntrySort.source,
+    'unread' => EntrySort.unread,
+    _ => EntrySort.date,
+  };
+}
+
+/// One post together with the site that published it.
+class RiverEntry {
+  const RiverEntry({
+    required this.siteUrl,
+    required this.siteTitle,
+    required this.url,
+    required this.title,
+    required this.date,
+    required this.readAt,
+  });
+
+  final String siteUrl;
+  final String? siteTitle;
+  final String url;
+  final String title;
+  final DateTime date;
+  final DateTime? readAt;
+
+  bool get unread => readAt == null;
+
+  /// The site as the reader knows it: its title, or its address until one is
+  /// recorded.
+  String get siteName => siteTitle ?? siteUrl;
+}