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

src/utun_instance.h

@@ -18,6 +18,8 @@ extern "C" {
 #include "firewall.h"
 #include "eim_nat.h"
 #include "nat_transport.h"
+#include "media_delivery/media_delivery.h"
+#include "media_async/media_async.h"
 #include "proxy/tcp_proxy_client.h"
 #include "etcp_router.h"
 #include "proxy/tcp_proxy_server.h"
@@ -144,6 +146,12 @@ struct UTUN_INSTANCE {
     // EIM NAT
     struct eim_nat_ctx nat;
     struct nat_transport_ctx nat_tr;
+
+    // Media delivery
+    struct media_delivery_ctx md;
+
+    // Media async engine (thread-per-task for crypto/file ops)
+    struct media_async* media_async;
     
     // Socket initialization status: 0=OK, 1=partial (some sockets failed), -1=error (none created)
     int socket_init_status;