feat: media_async + media_index modules, media_files table, tests

- media_async: thread-per-task async engine with primitives (sha256, sign, copy, uuid, file_size)
- media_index: media_files DB table with register_async for indexing chunks
- chat_msg: async media submission via register_async, simplified chat_msg_submit fields
- voice_recorder/attachment_sender: write single file (no chunk splitting on caller side)
- inputbar/messagelist: updated to new simplified submit interface
- test_media_async: 4-thread stress test (200K ops / 2 sec, 0 errors)
- test_media_index: 9 cases covering commit edge cases + async flow
- build: Makefile.am, CMakeLists.txt, utun_sources.cmake updated

tools/chatgui/src/messagedelegate.h

@@ -26,6 +26,7 @@ enum MessageDataRole {
     MsgVoiceNumBlocksRole = Qt::UserRole + 21,
     MsgVoiceSigsRole      = Qt::UserRole + 22,
     MsgFileDisplayNameRole = Qt::UserRole + 23,
+    MsgFileSizeRole       = Qt::UserRole + 24,
 };
 
 class MessageDelegate : public QStyledItemDelegate {
@@ -61,6 +62,7 @@ private:
         QRect emojiArea;
         bool isOutgoing = false;
         bool isVoice = false;
+        bool isFile = false;
         QRect voicePlayBtnRect;
         QRect voiceWaveformRect;
         QRect voiceDurationRect;