Add FolderDao and MailItemDao, update MailItem model to include folderId, fileId, size, messageId

src/db/dao/mailitemdao.h

@@ -0,0 +1,18 @@
+#pragma once
+
+#include "../databasemanager.h"
+#include "../../core/mailitem.h"
+#include <QVector>
+#include <optional>
+
+class MailItemDao
+{
+public:
+    static bool insert(const MailItem& item);
+    static bool update(const MailItem& item);
+    static bool remove(qint64 id);
+    static std::optional<MailItem> findById(qint64 id);
+    static QVector<MailItem> findAll();
+    static QVector<MailItem> findByFolderId(int folderId);
+    static QVector<MailItem> findByFolderIdSinceUid(int folderId, qint64 sinceUid);
+};