feat: add a stand-alone upscale endpoint to the server (#2026)

examples/server/runtime.h

@@ -2,6 +2,7 @@
 
 #include <algorithm>
 #include <cstdint>
+#include <filesystem>
 #include <mutex>
 #include <string>
 #include <vector>
@@ -43,6 +44,9 @@ struct UpscalerEntry {
     std::string fullpath;
     std::string model_name;
     int scale = 4;
+    int image_upscale_factor = 0;
+    uintmax_t file_size      = 0;
+    std::filesystem::file_time_type last_modified;
 };
 
 struct ServerRuntime {