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.c

@@ -509,6 +509,15 @@ void utun_instance_destroy(struct UTUN_INSTANCE *instance) {
     // Cleanup routing module (unbinds from etcp_router before etcp_router_destroy)
     routing_destroy(instance);
 
+    // Cleanup media delivery (unbinds from etcp_router before etcp_router_destroy)
+    if (instance->md.initialized) {
+        media_delivery_destroy(instance);
+    }
+
+    // Cleanup media async engine
+    media_async_destroy(instance->media_async);
+    instance->media_async = NULL;
+
     // Cleanup NAT (unbinds from etcp_router before etcp_router_destroy)
     if (instance->nat_tr.initialized) {
         nat_transport_destroy(instance);
@@ -636,6 +645,17 @@ int utun_instance_init(struct UTUN_INSTANCE *instance) {
             DEBUG_WARN(DEBUG_CATEGORY_NAT, "NAT transport init failed (non-fatal)");
         }
     }
+
+    // Initialize media delivery
+    if (media_delivery_init(instance) != 0) {
+        DEBUG_WARN(DEBUG_CATEGORY_GENERAL, "media_delivery init failed (non-fatal)");
+    }
+
+    // Initialize media async engine
+    instance->media_async = media_async_create();
+    if (!instance->media_async) {
+        DEBUG_ERROR(DEBUG_CATEGORY_GENERAL, "media_async create failed");
+    }
     
     // Note: TUN socket is already registered in tun_init()