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

app/lib/application/controllers/browser_controller.dart

@@ -277,6 +277,9 @@ class BrowserController {
         currentPage.value = page;
         error.value = null;
         visitedUrls.value = {...visitedUrls.value, url};
+        // Opening a post is reading it. Nothing happens when the address is
+        // not one, so this needs no knowledge of the reader's subscriptions.
+        await _db.markEntryRead(url);
         if (!notModified) {
           await _db.saveCachedPage(CachedPagesCompanion.insert(
             url: url,