server: (router) move model downloading to dedicated process (#24834)

* server: real-time model load progress tracking via /models/sse

* update docs

* server: move model download to child process

* rm unused

* fix most problems

* clean up

* nit fixes

* fix test case

* do not detact() thread

* shorter MODEL_DOWNLOAD_TIMEOUT in test

* throttle

common/arg.h

@@ -1,6 +1,7 @@
 #pragma once
 
 #include "common.h"
+#include "download.h"
 
 #include <set>
 #include <map>
@@ -133,7 +134,10 @@ void common_params_add_preset_options(std::vector<common_arg> & args);
 // return true if the model is ready to use
 // throw an exception if there is an error that prevents the model from being used (e.g. network error, model not found, etc)
 // if params.skip_download is true, no downloads will be attempted. return false if the model is invalid or missing (e.g. ETag check failed)
-bool common_params_handle_models(common_params & params, llama_example curr_ex);
+bool common_params_handle_models(
+    common_params & params,
+    llama_example curr_ex,
+    common_download_callback * callback = nullptr);
 
 // initialize argument parser context - used by test-arg-parser and preset
 common_params_context common_params_parser_init(common_params & params, llama_example ex, void(*print_usage)(int, char **) = nullptr);